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Duplicate listing sampling bias in visitor surveys
DUPLICATE LISTING SAMPLING B I A S IN V I S I T O R S U R V E Y S R i c h a r d R, P e r d u e North Carolina State University, USA
ABSTRACT T h e p u r p o s e of t h i s r e s e a r c h w a s to a s s e s s t h e s a m p l i n g bias in s t a t e a u t o m o b i l e visitor s u r v e y s a s s o c i a t e d w i t h two potential sources of duplicate listings in s a m p l i n g frames obtained w h e n distributing diary questionnaires to visitors at self-selected stops w i t h i n t h e s t a t e ' s b o u n d ary. A p a r t i c i p a t i o n bias occurs w h e n e v e r t h e visitor's probability of being s u r v e y e d varies as a f u n c t i o n of h i s / h e r p a r t i c i p a t i o n in a s a m p l i n g related activity. A visits bias occurs w h e n e v e r t h e i n d i v i d u a l ' s probability of being s u r v e y e d varies as a f u n c t i o n of t h e n u m b e r of t i m e s h e / s h e visits t h e s t a t e d u r i n g t h e s u r v e y period. Keywords: methodology, survey, sampling, visitor study, tourism.
Richard Perdae (Recreation Resources Administration, North Carolina State U n i v e r s i t y , R a l e i g h NC 2 7 6 9 5 , USA) r e c e i v e d h i s P h . D i n R e c r e a t i o n R e s o u r c e s D e v e l o p m e n t f r o m T e x a s A & M U n i v e r s i t y . T h e d a t a c o l l e c t i o n for t h i s a r t i c l e w a s c o n d u c t e d w h i l e t h e a u t h o r w a s a n a s s i s t a n t p r o f e s s o r a t t h e U n i v e r s i t y of Nebraska. A n n a l s o f Tourlsm Research, Vol. 13, pp. 261-278, 1986
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SAMPLINGBIASIN VISITOR SURVEYS RI~SUME; D e f o r m a t i o n de s o n d a g e p a r double c o m p t e d a n s des enq u e t e s de visiteurs. Le propos de cette r e c h e r c h e etait d ' e v a l u e r la d e f o r m a t i o n de s o n d a g e d a n s des e n q u e t e s p a r m i des p e r s o n n e s qui v i s i t a i e n t u n e t a t en voiture. Ces e n q u e t e s p r e s e n t a i e n t d e u x s o u r c e s e v e n t u e l l e s de ded o u b l e m e n t de compte d a n s les u n i t e s de s o n d a g e qui o n t ete o b t e n u e s q u a n d on a d i s t r i b u e des q u e s t i o n n a i r e s s u r des p r o d u i t s laitiers a des a r r e t s facultatifs a l ' i n t e r i e u r de l'etat. Une d e f o r m a t i o n de p a r t i c i p a t i o n a lieu c h e q u e fois ot~ la probabilite que le visiteur p a r t i c i p e r a d a n s l ' e n q u e t e varie en f o n c t i o n de sa p a r t i c i p a t i o n ~t u n e activite qui a u n r a p p o r t avec le sujet de l'enquete. Une d e f o r m a t i o n de f r e q u e n c e de visites a lieu c h a q u e fois oh la probabilite que l'on p a r t i c i p e r a & l ' e n q u e t e varie en f o n c t i o n du n o m b r e de fois o4 l'on visite l ' e t a t p e n d a n t la d u r e e de l'enquete. Mota clef: methodologie, e n q u e t e , sondage, e t u d e s u r les visiteurs, t o u r i s m e .
INTRODUCTION Because of its potential e c o n o m i c i m p o r t a n c e , s t a t e g o v e r n m e n t s h a v e become i n c r e a s i n g l y involved in travel a n d t o u r i s m p r o g r a m s (English 1981 ; K r e i s m a n 1982; P r i t c h a r d 1982). C o n c o m i t a n t w i t h t h i s i n v o l v e m e n t is a recognized n e e d for q u a l i t y m a r k e t r e s e a r c h , c o n c e r n i n g b o t h a c t u a l a n d potential s t a t e visitors (Bardon a n d Harding, 1981; Morucci 1980; Rovelstad a n d Blazer 1983). Of t h o s e s t a t e s r e s p o n d i n g to a 1982 survey, 70 p e r c e n t reported c o n d u c t i n g t o u r i s m m a r k e t r e s e a r c h ; 65 p e r c e n t of t h e s e s t a t e s also i n d i c a t e d t h e i r use of s u c h r e s e a r c h would i n c r e a s e in t h e f u t u r e (Rovelstad a n d Blazer 1983:4). As a major c o m p o n e n t of s t a t e t o u r i s m r e s e a r c h , visitor s u r v e y s are c o n d u c t e d e i t h e r a n n u a l l y or b i e n n i a l l y to m o n i t o r t h e c h a n g e s a n d t r e n d s in a s t a t e ' s t o u r i s m i n d u s t r y . By u s i n g a relatively s m a l l sample, t h e s u r v e y s focus on i d e n t i f y i n g t h e c h a r a c t e r i s t i c s a n d b e h a v i o r s of t h e visitor population, including, a m o n g o t h e r variables, t h e i r origins, d e s t i n a t i o n s , e x p e n d i t u r e s , behaviors, a n d socio-demographic c h a r a c t e r i s t i c s (Balden Associates 1978; Peat, Marwick, Mitchell a n d Co. 1976; U.S. D e p a r t m e n t of C o m m e r c e 1981 ; W e a v e r 1978). Increasingly, due to t h e p o t e n t i a l m e a s u r e m e n t p r o b l e m s a s s o c i a t e d w i t h e i t h e r on-site i n t e r v i e w s or mail q u e s t i o n n a i r e s , t h e s e s u r v e y s are being c o n d u c t e d u s i n g a diary 262
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questionnaire methodology in w h i c h respondents are given a questionnaire to be completed during their visit in the state and ret u r n e d by mail after they leave the state (e.g., Cournoyer and Kindahl 1983). Concern over the a c c u r a c y a n d representativeness of these surveys inspires continued methodological development, particularly of sampling procedures (Bailie and Gough 1976; Cournoyer and Kindhahl 1983; Huber and Rodger 1974). F u r t h e r research is needed which carefully examines alternative sampling methodologies involving not only w h e r e to sample travelers, but also how to select survey participants (e.g., Holland, Fedler and Ditton 1983), survey administration (e.g., Mills, Hodgson, McNeely and Masse 1981), and the causes and effects of survey nonresponse (e.g., Goodrich 1979). Of particular concern to visitor survey sampling procedures is the examination a n d improvement of the available sampling frames. Importantly, although a survey sampling frame is defined as the list of sampling units composing the study population from which the sample is selected (Babble 1983), very few state visitor survey samples are selected from a n actual list of potential sampling units. Rather, the sampling frame typically comprises a quasi-list of individuals or parties visiting designated sampling points during the survey period. In most cases, due to the inability to stop visitors at ports-of-entry to the state, these sampling points are places spread throughout the state where visitors self-select to stop (e.g., interstate rest areas, gasoline service stations, attractions, etc.). For example, if a visitor survey is being conducted by contacting visitor parties at interstate rest areas during a three m o n t h period, the sampling frame comprises all visitor parties stopping at a n interstate rest area during the specified three months. The potential problems with such sampling frames can be grouped into two categories, incomplete coverage of the study population (e.g., Lucas 1975; Lucas and Oltman 1971; Scotler 1981) and duplicate listings of some sampling units (e.g., Perdue and Ditton 1983; Rao 1968). The purpose of the r e s e a r c h reported in this article is to examine two forms of duplicate listings which are potential sources of sampling bias in state visitor surveys involving the distribution of diary questionnaires to visitors at self-selected stops. First, a duplicate listing bias occurs w h e n e v e r the visitor's probability of being sampled varies as a function of his/her frequency of participation in a sampling related activity. If visitors are being contacted at a particular type of area (e.g., interstate rest areas), each visit to such a n area is a sampling frame listing. Multiple sampling frame listings 1986 ANNALS OF TOURISM RESEARCH
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SAMPLING BIAS IN VISITOR SURVEYS
m a y , therefore, exist for t h o s e people w h o visit s u c h a r e a s several t i m e s d u r i n g t h e i r s t a y in t h e s t u d y area. While it is n o t t e c h n i c a l l y correct, t h i s bias h a s b e e n t e r m e d a " l e n g t h - o f - s t a y " bias in b o t h t h e r e c r e a t i o n a n d t o u r i s m l i t e r a t u r e (Archer a n d S h e a 1975; L u c a s 1963; W u r s t 1955). It is incorrect in t h a t while t h e y are typically h i g h l y correlated, p a r t i c i p a t i o n in t h e s a m p l i n g related activity a n d t h e a m o u n t of time s p e n t in t h e s t u d y a r e a are n o t n e c e s s a r i l y t h e same. It is possible for a n individual to visit a n a r e a for a n e x t e n d e d period w i t h o u t visiting a s a m p l i n g c o n t a c t point a n d to visit a cont a c t point several t i m e s in a relatively s h o r t visit. C o n s e q u e n t l y , in t h i s paper, t h i s bias will be t e r m e d a " p a r t i c i p a t i o n b i a s . " Visitor e x p e n d i t u r e s are p e r h a p s t h e m o s t significant problem a s s o c i a t e d w i t h a p a r t i c i p a t i o n bias (Archer a n d S h e a 1975). Since participation in t h e s a m p l i n g related activity does t e n d to i n c r e a s e w i t h l e n g t h of s t a y w h i c h , in t u r n , t e n d s to be related to e x p e n d i t u r e s , t h i s bias typically r e s u l t s in a t e n d e n c y to o v e r - e s t i m a t e b o t h average visitor p a r t y e x p e n d i t u r e s a n d t h e economic i m p a c t of t o u r i s m . Second, a duplicate listing bias also occurs w h e n e v e r a visitor's s a m p l i n g probability varies as a f u n c t i o n of t h e n u m b e r of t i m e s h e / s h e visits t h e s t a t e d u r i n g t h e s u r v e y period (Absher a n d Samd a h l 1983; Babbie 1983; W a g a r a n d T h a l h e i m e r 1968). A s s u m i n g no p a r t i c i p a t i o n bias, e a c h visit by i n d i v i d u a l s m a k i n g multiple trips to a s t a t e d u r i n g t h e s u r v e y period is a s a m p l i n g f r a m e listing. C o n s e q u e n t l y , t h e probability of being s u r v e y e d for t h e s e individu a l s is s u b s t a n t i a l l y h i g h e r t h a n t h a t for t h o s e i n d i v i d u a l s w h o only m a k e one trip to t h e state. T h e i m p o r t a n c e of t h i s bias, t e r m e d in t h e r e m a i n d e r of t h i s p a p e r as a " v i s i t s b i a s , " d e p e n d s u p o n t h e u n i t of a n a l y s i s u s e d in a s s e s s i n g t h e s u r v e y r e s u l t s (Babbie 1983:146; Kish 1965). If t h e u n i t of a n a l y s i s is t h e trip, i.e., describing or c o m p a r i n g trip c h a r a c t e r i s t i c s s u c h as e x p e n d i t u r e s , l e n g t h of stay, a n d trip purpose, a visits bias is n o t relevant. However, in t h o s e s i t u a t i o n s w h e r e t h e visitor is t h e u n i t of a n a l y s i s , failure to correct for a visits bias r e s u l t s in a t e n d e n c y to over-repres e n t t h e more f r e q u e n t visitor. E x a m p l e s of s u c h a n a l y s e s include a s s e s s m e n t s of visitor c h a r a c t e r i s t i c s (e.g., origin, socio-demographics, a n d a t t i t u d e s / i m a g e s / p r e f e r e n c e s ) . As i n d i c a t e d by t h e previous citations, articles c o n c e r n i n g t h e s e b i a s e s h a v e b e e n p u b l i s h e d in b o t h t h e r e c r e a t i o n a n d t o u r i s m literature. Several a u t h o r s h a v e p u b l i s h e d empirical d a t a c o n c e r n ing t h e effects of a p a r t i c i p a t i o n bias on e s t i m a t e s of b o t h trip a n d visitor c h a r a c t e r i s t i c s (Archer a n d S h e a 1975; L u c a s 1963). However, s y s t e m a t i c a n a l y s e s of t h e potential visits bias in visitor surveys, i n c l u d i n g its e x t e n s i t y a n d effects on s u r v e y r e s u l t s b o t h indi264
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RICHARDPERDUE vidually a n d in c o m b i n a t i o n w i t h t h e effects of t h e potential p a r t i c i p a t i o n bias, h a v e n o t b e e n published. T h e p u r p o s e of t h i s r e s e a r c h w a s to c o n d u c t s u c h a n a n a l y s i s . METHODOLOGY During t h e s u m m e r of 1983 (May 2 7 t h to S e p t e m b e r 5th), d a t a were collected by d i a r y q u e s t i o n n a i r e from n o n r e s i d e n t a u t o m o b i l e visitors to Nebraska. While requiring visitors to stop at port-ofe n t r y roadblocks (e.g., H u n t a n d Cadez 1978) h a s obvious s a m p l i n g merits, t h i s s a m p l i n g procedure w a s rejected by t h e N e b r a s k a Dep a r t m e n t of Roads, b e c a u s e of t h e potential s a f e t y a n d liability problems. T h e c o m m o n a l t e r n a t i v e of s u r v e y i n g visitors at port-ofe n t r y visitor i n f o r m a t i o n c e n t e r s (e.g., Gitelson a n d C r o m p t o n 1983) w a s rejected due to t h e potential s a m p l i n g bias associated w i t h t h e inability to s u r v e y b o t h t h o s e individuals traveling on t h e i n t e r s t a t e , b u t not stopping at t h e visitor i n f o r m a t i o n c e n t e r s a n d t h o s e i n d i v i d u a l s not t r a v e l i n g on t h e i n t e r s t a t e . C o n s e q u e n t l y , visitors were c o n t a c t e d at gasoline service s t a t i o n s located t h r o u g h o u t t h e state. S a m p l i n g d a y s (102) were allocated to t w e n t y - e i g h t s u r v e y c o m m u n i t i e s on t h e basis of h i g h w a y traffic flows a n d t h e n u m b e r of h o t e l / m o t e l r o o m s in e a c h c o m m u n i t y . S a m p l i n g s t a t i o n s w i t h i n e a c h c o m m u n i t y were selected on t h e b a s i s of a t e l e p h o n e s u r v e y of local hotel/motel m a n a g e r s , essentially a s k i n g t h e m to identify t h e major s t a t i o n s for n o n r e s i d e n t visitors in t h e i r c o m m u n i t y . A m e m b e r of e a c h visitor p a r t y s t o p p i n g at t h e selected s t a t i o n s on a n a s s i g n e d s a m p l i n g d a y w a s a s k e d to participate in t h e survey. Individuals w h o agreed to participate in t h e s u r v e y were a s k e d to provide t h e i r n a m e a n d a d d r e s s on a c o n t a c t card. T h e y were t h e n given a d i a r y q u e s t i o n n a i r e to complete d u r i n g t h e i r visit. E a c h s u r v e y p a r t i c i p a n t w a s s e n t a p o s t c a r d r e m i n d e r to r e t u r n t h e q u e s t i o n n a i r e s e v e n d a y s after t h e initial contact. N o n r e s p o n d e n t s were s e n t a follow-up letter a n d a s e c o n d copy of t h e q u e s t i o n n a i r e 14 d a y s a f t e r t h e initial contact. Overall, 3782 visitor p a r t i e s were contacted, from w h i c h 3 6 3 4 individuals (96.1%) agreed to participate in t h e s u r v e y a n d 2345 individuals (62.2%) completed a n d r e t u r n e d t h e d i a r y q u e s t i o n n a i r e . Since t h e s a m p l i n g f r a m e comprised all visitor p a r t i e s e n t e r i n g gasoline service stations, potential p a r t i c i p a t i o n bias w a s m e a s u r e d by a q u e s t i o n a s k i n g respond e n t s how m a n y t i m e s t h e y stopped at gasoline s t a t i o n s d u r i n g t h e i r N e b r a s k a visit. For t h e p u r p o s e of t h i s a n a l y s i s , a r a n d o m s a m p l e of 400 d i a r y 1986 ANNALSOF TOURISMRESEARCH
265
SAMPLINGBIASIN VISITOR SURVEYS q u e s t i o n n a i r e r e s p o n d e n t s w a s s u r v e y e d in a follow-up s t u d y cond u c t e d d u r i n g October, 1983. T h e s e individuals were s e n t a postc a r d q u e s t i o n n a i r e a s k i n g t h e m h o w m a n y t i m e s t h e y visited Neb r a s k a d u r i n g t h e s u m m e r (May 2 7 t h to S e p t e m b e r 5th) of 1983 a n d t h e d i s t a n c e from t h e i r h o m e to Nebraska. I m p o r t a n t l y , tourism in N e b r a s k a is d o m i n a t e d by t r a v e l - t h r o u g h visitors; 74.5 perc e n t of t h e i n d i v i d u a l s c o n t a c t e d d u r i n g t h e s u m m e r of 1983 were t r a v e l i n g t h r o u g h t h e s t a t e e n r o u t e to a n o t h e r area. A large perc e n t a g e of t h e s e visitors v a r y t h e i r r o u t e s for t h e different legs of t h e i r trip. For example, m a n y visitors travel to t h e i r d e s t i n a t i o n t h r o u g h N e b r a s k a on I n t e r s t a t e 80, b u t r e t u r n h o m e e i t h e r t h r o u g h S o u t h D a k o t a on I n t e r s t a t e 90 or t h r o u g h K a n s a s on I n t e r s t a t e 70. Similarly, m a n y visitors travel to t h e i r d e s t i n a t i o n t h r o u g h e i t h e r S o u t h D a k o t a or K a n s a s a n d r e t u r n h o m e t h r o u g h Nebraska. As a r e s u l t of t h i s b e h a v i o r a l p a t t e r n , r e s p o n d e n t s to t h e d i a r y questionn a i r e were i n s t r u c t e d to complete t h e i n s t r u m e n t only for t h a t leg of t h e i r trip on w h i c h t h e y were c o n t a c t e d . A l t h o u g h it u n d o u b t e d l y inflated t h e n u m b e r of visits m e a s u r e , r e s p o n d e n t s to t h e follow-up s u r v e y were, c o n s e q u e n t l y , a s k e d t h r e e questions: h o w m a n y t i m e s t h e y traveled to N e b r a s k a d e s t i n a t i o n s d u r i n g t h e s u m m e r of 1983, h o w m a n y t i m e s t h e y t r a v e l e d t h r o u g h t h e s t a t e e n r o u t e from t h e i r h o m e to a d e s t i n a t i o n in a n o t h e r state, a n d h o w m a n y t i m e s t h e y t r a v e l e d t h r o u g h t h e s t a t e on t h e i r w a y h o m e from a d e s t i n a t i o n . T h e total n u m b e r of visits w a s c a l c u l a t e d by a d d i n g t h e r e s p o n s e to t h e s e t h r e e questions. Using a p o s t c a r d r e m i n d e r a n d one follow-up mailing, 290 i n d i v i d u a l s completed a n d r e t u r n e d t h e p o s t c a r d q u e s t i o n n a i r e , a r e s p o n s e rate of 72.5 percent. For t h e p u r p o s e of a s s e s s i n g t h e p a r t i c i p a t i o n a n d visits s a m p l i n g biases, t h e d i a r y q u e s t i o n n a i r e d a t a for t h e s e i n d i v i d u a l s were m e r g e d w i t h t h e follow-up s u r v e y d a t a to create a final d a t a set c o m p r i s i n g 290 c a s e s a n d i n c l u d i n g both s a m p l i n g bias m e a s u r e s a n d t h e visitor a n d t h e trip c h a r a c t e r i s t i c d a t a from t h e d i a r y q u e s t i o n n a i r e . Since is w a s n o t possible, due to f i n a n c i a l c o n s t r a i n t s , to c o n d u c t t h e follow-up s u r v e y w i t h all r e s p o n d e n t s to t h e d i a r y questionnaire, t h e following r e s u l t s c a n n o t be c o n s i d e r e d r e p r e s e n t a t i v e of t h e N e b r a s k a visitor population. As s h o w n in Table 1, t h e r e were potential differences in t h e visitor a n d trip c h a r a c t e r i s t i c s of t h e follow-up s u r v e y r e s p o n d e n t s as c o m p a r e d to all visitor s u r v e y res p o n d e n t s , p r o b a b l y r e s u l t i n g from a c o m b i n a t i o n of b o t h s a m p l i n g error a n d n o n r e s p o n s e s a m p l i n g bias. T h e d a t a were, however, c o n s i d e r e d acceptable to e x a m i n e t h e potential p a r t i c i p a t i o n a n d visits s a m p l i n g bias a s s o c i a t e d w i t h t h e d i a r y q u e s t i o n n a i r e m e t h odology. 266
1986 ANNALSOF TOURISMRESEARCH
RICHARD PERDUE Table 1
C o m p a r i s o n o f Nebraska Visitor S u r v e y a n d F o l l o w - u p Survey Results Variable
N e b r a s k a Visitor S u r v e y
Follow-up S u r v e y
42.4
44.9
46.0% 54.0
48.4% 51.6
30.8% 30.8 38.4
31.8% 29.3 38.9
19.8% 80.2
20.8% 79.2
18.3% 17.8 25.3 19.1 19.6
16.7 % 21.2 25.7 17.0 19.4
38.6%
37.2%
19.4
19.0
8.6
10.3
Visitor Characteristics 1. Age" 2. Sex Male Female 3. E d u c a t i o n High school or less Some college College g r a d u a t e 4. E x p o s u r e to N e b r a s k a T o u r i s m Ads Yes No
Trip Characteristics 1. Trip P l a n n i n g Period Less t h a n two w e e k s Two to four w e e k s One to t h r e e m o n t h s F o u r to six m o n t h s Over six m o n t h s 2. P r i m a r y Trip P u r p o s e Visiting family a n d friends Recreation and sightseeing Business and convention All o t h e r p u r p o s e s 3. Days in N e b r a s k a " 4. E x p e n d i t u r e s a 5. A t t r a c t i o n Visits a 6. Highway Rest A r e a Visits a
33.4 2.26 122.50 0.40 2.32
33.4 2.17 121.60 0.36 2.35
a mean values
RESULTS The frequency distributions for the sampling measures are pres e n t e d i n T a b l e 2. T h e f o l l o w - u p s u r v e y r e s p o n d e n t s a v e r a g e d 2 . 2 4 trips to or through Nebraska during the summer of 1983 and, on the 1986 ANNALS OF TOURISM RESEARCH
267
SAMPLING BIAS IN VISITOR SURVEYS
Table 2 F r e q u e n c y Distribution o f Gasoline Station Stops and Visits to Nebraska Visits to Nebraska Frequency
Visitors freq. pct.
One Two Three Four Five Six or more Total
109 106 41 18 5 11 290
Visits freq. pct.
37.6 109 36.6 212 14.1 123 6.2 72 1.7 25 3.8 109 100.0 650 ~ = 2.24
16.8 32.6 18.9 11.1 3.8 16.8 100.0
Gasoline Station Stops Visitors freq. pct. 81 123 45 16 7 4 276
Gas Stops freq. pct.
29.3 81 44.6 246 16.3 135 5.8 64 2.5 35 1.5 26 100.0 587 ~ = 2.13
13.8 41.6 23.0 10.9 4.4 4.4 100.0
s u r v e y e d trip, 2.13 stops at gasoline service s t a t i o n s . Over 60 perc e n t of t h e r e s p o n d e n t s m a d e more t h a n one trip to t h e state. Similarly, a l m o s t 71 p e r c e n t of t h e r e s p o n d e n t s m a d e more t h a n one stop at a gasoline service s t a t i o n . Of t h e reported 650 N e b r a s k a visits, 50.6 p e r c e n t were m a d e by f r e q u e n t visitors (-> 3 trips to t h e state). F r e q u e n t visitors, however, r e p r e s e n t e d only 25.8 p e r c e n t of t h e r e s p o n d e n t s . Of t h e reported 587 stops a t gasoline service stations, 260 (44.3%) were by f r e q u e n t s t o p p e r s (-> 3 stops), 26.1 perc e n t of t h e r e s p o n d e n t s . S u b s t a n t i a l potential exists, therefore, for duplicate listings in t h e visitor s u r v e y s a m p l i n g frame. Given t h i s p o t e n t i a l duplicate listing problem, a n a l y s e s were n e x t c o n d u c t e d of t h e r e l a t i o n s h i p b e t w e e n t h e selected visitor c h a r a c t e r i s t i c s a n d trips to N e b r a s k a a n d n u m b e r of gasoline station stops v a r i a b l e s (Table 3). Trips to N e b r a s k a varied w i t h age, origin ( m e a s u r e d on t h e follow-up p o s t c a r d q u e s t i o n n a i r e as t h e d i s t a n c e from t h e i n d i v i d u a l ' s h o m e to Nebraska), a n d w h e t h e r or n o t t h e individual h a d s e e n or h e a r d N e b r a s k a t o u r i s m advertisem e n t s prior to leaving home. F u r t h e r a n a l y s i s identified significant i n t e r r e l a t i o n s h i p s b e t w e e n t h e s e variables w h i c h e m p h a s i z e d t h e i m p o r t a n c e of t h e d i s t a n c e from h o m e to N e b r a s k a m e a s u r e . Specifically, N e b r a s k a ' s t o u r i s m p r o m o t i o n a l efforts are focused on a geographical m a r k e t w i t h i n 500 miles of t h e s t a t e ' s border. Conseq u e n t l y , as d i s t a n c e from t h e s t a t e i n c r e a s e d , e x p o s u r e to N e b r a s k a t o u r i s m a d v e r t i s e m e n t s decreased. Similarly, as d i s t a n c e from Neb r a s k a increased, visitor age also decreased. T h e n u m b e r of gasoline s t a t i o n stops w a s significantly related only to t h e e d u c a t i o n a l 268
1 9 8 6 ANNALS OF TOURISM R E S E A R C H
RICHARD PERDUE Table 3
Visits to Nebraska and Gasoline Station Stops by Visitor Characteristics Visits to Nebraska Visitor Characteristic
Mean
Age Sex Male Female Education High school Some college College graduate Distance from Home to Nebraska Exposure to Nebraska Tourism Ads Yes No
Statistic
Gasoline Station Stops Mean
r = .07 F = 0.25
r = .14" F = 0.73 2.15 2.08
2.28 2. I 0
F = 4.06"
F = 1.90 2.32 2.18 1.89
2.46 2.18 1.96 r = --. 14"
r=
F = 8.02 b 2.87 2.09
Statistic
.11
F---- 1.55
1.95 2.15
a significant at alpha ---.05 b significant at alpha <-.01
level of t h e r e s p o n d e n t s . I n d i v i d u a l s w i t h h i g h e r l e v e l s of e d u c a t i o n t e n d e d to s t o p a t g a s o l i n e s t a t i o n s l e s s f r e q u e n t l y t h a n t h o s e w i t h less education. S i m i l a r a n a l y s e s w e r e c o n d u c t e d to e x a m i n e t h e r e l a t i o n s h i p bet w e e n s e l e c t e d t r i p c h a r a c t e r i s t i c s a n d t h e n u m b e r of g a s o l i n e s t a t i o n s t o p s m e a s u r e ( T a b l e 4). A s e x p e c t e d , t h e n u m b e r of g a s o l i n e s t a t i o n s t o p s w a s p o s i t i v e l y r e l a t e d to b o t h t h e n u m b e r of d a y s a n d t h e a m o u n t o f m o n e y s p e n t in t h e s t a t e . S i n c e b o t h g a s o l i n e s e r v i c e s t a t i o n s a n d h i g h w a y rest a r e a s serve as r e s t r o o m s a n d s o u r c e s of i n f o r m a t i o n for t o u r i s t s , t h e s t r e n g t h o f t h e r e l a t i o n s h i p b e t w e e n those two variables was not expected. Further analysis indicated t h a t r e s t a r e a v i s i t s w e r e s i g n i f i c a n t l y r e l a t e d to t h e n u m b e r of d a y s s p e n t in t h e s t a t e . H e n c e , t h e o b s e r v e d r e s t a r e a v i s i t s to g a s o l i n e station stops relationship was probably spurious. Similarly, the o b s e r v e d r e l a t i o n s h i p t h a t t h e l e n g t h of t h e t r i p p l a n n i n g p e r i o d d e c r e a s e d w i t h t h e n u m b e r of g a s s t o p s w a s n o t e x p e c t e d . A s s u m i n g t h a t t h e t r i p p l a n n i n g p e r i o d w o u l d l e n g t h e n w i t h t h e n u m b e r of d a y s in N e b r a s k a , it w a s a l s o e x p e c t e d to l e n g t h e n w i t h t h e n u m b e r 1986 ANNALS OF TOURISM RESEARCH
269
S A M P L I N G BIAS IN V I S I T O R S U R V E Y S
Table 4
Gasoline Station Stops by Trip Characteristics Gasoline Station Stops Trip Characteristic Trip P l a n n i n g Period Less t h a n two w e e k s Two to four w e e k s One to three m o n t h s Four to six m o n t h s Over six m o n t h s P r i m a r y Trip Purpose Visiting family a n d friends Recreation a n d Sightseeing Business a n d convention All other p u r p o s e s Days in N e b r a s k a Expenditures Attraction Visits Highway Rest Area Visits
Mean
Statistic F = 2.57"
2.40 2.33 2.00 1.98 1.84 F = 0.60 2.31 2.13 2.09 1.98 r---r= r= r=
.19 b .32 b .03 .26 b
a s i g n i f i c a n t a t a l p h a --- . 0 5 b s i g n i f i c a n t a t a l p h a -< .001
of g a s o l i n e s t a t i o n s t o p s . T h e r e w a s , h o w e v e r , n o r e l a t i o n s h i p b e t w e e n t h e l e n g t h of t h e t r i p p l a n n i n g p e r i o d a n d t h e n u m b e r of d a y s s p e n t in N e b r a s k a , p r o b a b l y b e c a u s e of t h e l a r g e p e r c e n t a g e of N e b r a s k a v i s i t o r s t r a v e l i n g t h r o u g h t h e s t a t e e n r o u t e to a n o t h e r d e s t i n a t i o n . T h e n u m b e r of d a y s s p e n t in N e b r a s k a w a s n o t r e l a t e d to t h e o v e r a l l l e n g t h of t h e v i s i t o r ' s t r i p . Correcting an existing data base for a sampling bias resulting f r o m d u p l i c a t e l i s t i n g s in t h e s a m p l i n g f r a m e r e q u i r e s t h e d e v e l o p m e n t of a w e i g h t i n g f a c t o r w h i c h is i n v e r s e l y p r o p o r t i o n a l to t h e visitor's selection probability (Cochran 1977; Cummings 1979; K a l t o n 1 9 8 3 ; S u d m a n 1976). If t h e v i s i t o r ' s s e l e c t i o n p r o b a b i l i t y Pl is k n o w n , a w e i g h t i n g c o e f f i c i e n t of k / p i c a n b e u s e d , w h e r e k is a c o n v e n i e n t c o n s t a n t . T h e o b v i o u s c h o i c e of k = 1, s o t h a t t h e w e i g h t wl ---- 1/p~, is p a r t i c u l a r l y u s e f u l w h e n e s t i m a t e s of p o p u l a t i o n t o t a l s a r e n e e d e d . F o r e x a m p l e , w i t h v i s i t o r e x p e n d i t u r e s el, Ew~e, p r o v i d e s t h e t o t a l t o u r i s m e x p e n d i t u r e f o r t h e v i s i t o r p o p u l a t i o n . H o w e v e r , s i n c e t h e p~ in v i s i t o r s t u d i e s a r e f r e q u e n t l y v e r y s m a l l v a l u e s , k ---- 1 t y p i c a l l y r e s u l t s in n o n - i n t e g e r w e i g h t s w h i c h r e q u i r e s e v e r a l d e c i m a l p l a c e s a n d a r e difficult to u s e w i t h s o m e 270
1986 ANNALS OF TOURISM RESEARCH
RICHARD PERDUE
c o m p u t e r s t a t i s t i c a l packages. C o n s e q u e n t l y , a n a l t e r n a t i v e value of k is f r e q u e n t l y u s e d in visitor s t u d i e s to simplify t h e w e i g h t s a n d t h e i r application. A l t h o u g h e s t i m a t e s of p o p u l a t i o n p a r a m e t e r s s u c h as m e a n , v a r i a n c e , a n d p e r c e n t a g e d i s t r i b u t i o n s will still be correct w i t h v a l u e s o t h e r t h a n k ---- 1, s u m m e d v a l u e s s u c h as Ew~e~ m u s t be divided by k to derive e s t i m a t e s of p o p u l a t i o n totals. Since in m o s t visitor s t u d i e s p~ is u n k n o w n b u t a s s u m e d to be equal for all visitors, a w e i g h t i n g coefficient o t h e r t h a n k/pl is needed. Given t h a t differential s a m p l i n g probabilities r e s u l t in visitor s u r v e y s from duplicate listings in t h e s a m p l i n g f r a m e a s s o c i a t e d w i t h t h e visitor's p a r t i c i p a t i o n in a s a m p l i n g related activity 11a n d n u m b e r of visits to t h e s t a t e d u r i n g t h e s u r v e y period v~, k/l~ a n d k/v~ provide w e i g h t s w h i c h c a n be u s e d to correct visitor s u r v e y d a t a for t h e two s a m p l i n g f r a m e problems, respectively. For t h o s e variables i n f l u e n c e d by duplicate listings a s s o c i a t e d w i t h both p a r t i c i p a t i o n a n d visits, w~--(k/ll) + (k/v~) provides t h e a p p r o p r i a t e w e i g h t i n g c o e f f c i e n t (Wagar a n d T h a l h e i m e r , 1968). Again, e s t i m a t e s of p o p u l a t i o n p a r a m e t e r s s u c h as m e a n , v a r i a n c e , a n d p e r c e n t a g e d i s t r i b u t i o n s will be correct w i t h t h e s e w e i g h t i n g coefficients. Population totals are e s t i m a t e d by correcting for t h e e s t i m a t e d n u m b e r of visitors to t h e s t a t e d u r i n g t h e s u r v e y period. For example, a n e s t i m a t e of total visitor e x p e n d i t u r e s would be: N
~ wlel i~l
N
W
where wl -- p a r t i c i p a t i o n w e i g h t i n g coeffcient, el = visitor e x p e n d i t u r e , N---- s a m p l e size, a n d V ----e s t i m a t e d n u m b e r of s t a t e visitors d u r i n g t h e s u r v e y period. Using t h e s e f o r m u l a s , w e i g h t e d v a l u e s for t h e selected trip a n d visitor c h a r a c t e r i s t i c s of t h e N e b r a s k a Visitor S u r v e y were calculated for t h e follow-up s u r v e y r e s p o n d e n t s a n d are p r e s e n t e d , along w i t h t h e u n w e i g h t e d values, in Tables 5 a n d 6. C o m p a r i s o n of t h e w e i g h t e d a n d u n w e i g h t e d visitor c h a r a c t e r i s tic v a l u e s revealed potential differences, p a r t i c u l a r l y on t h e dist a n c e from h o m e to N e b r a s k a a n d e x p o s u r e to N e b r a s k a t o u r i s m a d v e r t i s e m e n t s m e a s u r e s {Table 5). However, t h e effects of t h e participation a n d visits b i a s e s t e n d e d to offset e a c h o t h e r in s u c h a w a y t h a t t h e c o m b i n a t i o n of t h e two w e i g h t i n g factors r e s u l t e d in v a l u e s 1986 ANNALS OF TOURISM RESEARCH
271
S A M P L I N G BIAS IN V I S I T O R S U R V E Y S
Table 5
Weighted and Unweighted Visitor Characteristic Values Values Weighted by
Visitor Characteristic Agea Sex Male Female Education High school or less Some college College graduate Distance from home to Nebraska a Exposure to Nebraska Tourism Advertisements Yes No
Unweighted Values
Nebraska Visits
Gasoline Station Stops
Gasoline Station Stops and Nebraska Visits
44.9
44.1
44.5
44.2
48.4% 51.6
47.4% 52.6
47.3% 52.7
47.6% 52.4
31.8% 29.3 38.9 565.7
29.7% 28.5 41.8 591.9
29.7% 27.4 42.9 537.0
28.9% 28.4 42.7 567.3
20.8% 79.2
18.4% 81.6
23.4% 76.6
21.3% 78.7
"mean values
o n l y slightly d i f f e r e n t f r o m t h e u n w e i g h t e d val ues. C o n s e q u e n t l y , w hile r e p l i c a t i o n s w i t h a d d i t i o n a l d a t a b a s e s a r e n e c e s s a r y , it app e a r s t h a t t h e visitor c h a r a c t e r i s t i c s of t h e follow-up s u r v e y d a t a base were not biased w h e n comparing the unweighted values with t h o s e w e i g h t e d to c o r r e c t for b o t h t h e p a r t i c i p a t i o n a n d visits b i as es . H o w e v e r , c o r r e c t i o n for o n l y o n e of t h e t w o p o t e n t i a l duplic a t e listing b i a s e s m a y r e s u l t in s u b s t a n t i a l l y i n c o r r e c t v a l u e s , p a r t i c u l a r l y o n m e a s u r e s a s s o c i a t e d w i t h vi s it or origin a n d e x p o s u r e to s t a t e p r o m o t i o n a l efforts. C o m p a r i s o n of t h e u n w e i g h t e d a n d w e i g h t e d v a l u e s in T a b l e 6 reflects t h e p o t e n t i a l i m p o r t a n c e of c o r r e c t i n g vi si t or s u r v e y t ri p c h a r a c t e r i s t i c d a t a for a possible p a r t i c i p a t i o n bias. T h e w e i g h t e d v a l u e s for d a y s in N e b r a s k a , e x p e n d i t u r e s , a n d n u m b e r of r e s t a r e a visits a p p e a r s u b s t a n t i v e l y s m a l l e r t h a n t h e u n w e i g h t e d val ues, p o t e n t i a l l y i n d i c a t i n g t h e bi a s in r e p o r t i n g u n w e i g h t e d data. T h e u n w e i g h t e d d a y s a n d e x p e n d i t u r e s v a l u e s a r e 11.8 a n d 11.5 perc e n t g r e a t e r t h a n t h e w e i g h t e d val ues , r e s p e c t i v e l y . Similarly, 272
1986 ANNALS OF TOURISM RESEARCH
RICHARDPERDUE Table 6
Weighted and Unweighted Trip Characteristic Values Trip Characteristic Trip Planning Period Less than two weeks Two to four weeks One to three months Four to six months Over six months Primary Trip Purpose Visiting family and friends Recreation and sightseeing Business and convention All other purposes Days in Nebraska a Expenditures" Attraction Visits a Highway Rest Area Visits a
Unweighted Values
Values Weighted by Number of Gasoline Station Stops
16.7% 21.2 25.7 17.0 19.4
18.9% 21.7 26.5 15.9 17.0
37.2% 19.0 10.3 33.4 2.17 121.60 0.36 2.35
37.0% 19.9 10.0 32.3
1.94 109.01 0.33 2.20
a mean values
w h e n c o r r e c t e d for t h e n u m b e r of g a s o l i n e s t a t i o n stops, visitor t ri p p l a n n i n g a p p e a r s s h o r t e r t e r m t h a n i n d i c a t e d by t h e u n w e i g h t e d data. CONCLUSIONS S a m p l i n g v is i t or s to a large g e o g r a p h i c a l a r e a w i t h r e l a t i v e l y o p e n a c c e s s f r o m a v a r i e t y of p o i n t s is a n e x t r e m e l y c o m p l e x m e t h odological p r o b l e m . In all b u t a few cas e s , in s t a t e visitor s u r v e y s t h i s p r o b l e m is f u r t h e r m a g n i f i e d by t h e i n a b i l i t y to stop vi si t ors at p o r t s - o f - e n t r y , d u e to s a f e t y a n d liability c o n c e r n s . T h e p u r p o s e of t h i s r e s e a r c h w a s to e x a m i n e t h e p o t e n t i a l s a m p l i n g bi as assoc i a t e d w i t h two s o u r c e s of d u p l i c a t e listings in t h e s a m p l i n g f r a m e o b t a i n e d w h e n d i s t r i b u t i n g d i a r y q u e s t i o n n a i r e s to vi si t ors at selfselected stops within the state's boundary. A potential participat i o n b ias e x i s t s w h e n e v e r t h e v i s i t o r ' s p r o b a b i l i t y of b e i n g s a m p l e d v a r i e s as a f u n c t i o n of h o w f r e q u e n t l y h e / s h e p a r t i c i p a t e s in a specific activity. A p o t e n t i a l visits bi as e x i s t s w h e n e v e r t h e individu a l ' s p r o b a b i l i t y of b e i n g s a m p l e d v a r i e s as a f u n c t i o n of h o w freq u e n t l y h e / s h e visits t h e s t a t e d u r i n g t h e s u r v e y period. 1986 ANNALSOF TOURISM RESEARCH
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SAMPLINGBIASIN VISITOR SURVEYS Prior to m a k i n g a n y c o n c l u s i o n s c o n c e r n i n g t h e r e s u l t s of t h i s s t u d y , it is i m p o r t a n t to m a k e t h r e e points. First, in order to a s s e s s t h e effects of a visits bias, it is n e c e s s a r y to c o n d u c t a follow-up s u r v e y w i t h r e s p o n d e n t s to t h e d i a r y q u e s t i o n n a i r e . To d e t e r m i n e t h e n u m b e r of visits to t h e s t a t e d u r i n g t h e entire visitor s u r v e y period, t h i s follow-up s u r v e y m u s t be c o n d u c t e d following t h e e n d of t h a t period. It is u n r e a s o n a b l e to expect a complete r e s p o n s e to t h i s follow-up survey. C o n s e q u e n t l y , t h e potential n o n r e s p o n s e bias in t h e follow-up s u r v e y r e s u l t s m a y limit t h e i r value in correcting t h e visitor s u r v e y data. A l t h o u g h several a u t h o r s h a v e c o m m e n t e d on t h e potential for a visits bias in visitor s u r v e y r e s e a r c h , previous d a t a c o n c e r n i n g its effects h a v e n o t b e e n published, p r o b a b l y bec a u s e of t h e e x p e n s e a n d difficulty a s s o c i a t e d w i t h t h i s follow-up survey. A p r i m a r y p u r p o s e of t h i s methodological note is to provide s u c h data. Due to t h e a s s o c i a t e d expense, however, it w a s necess a r y to c o n d u c t t h e follow-up s u r v e y w i t h only a s m a l l s a m p l e of t h e 1983 N e b r a s k a Visitor S u r v e y r e s p o n d e n t s . C o n s e q u e n t l y , alt h o u g h t h e r a w d a t a v a l u e s for t h e follow-up s u r v e y were s i m i l a r to t h o s e for t h e overall visitor survey, t h e c o m b i n a t i o n of t h e previously m e n t i o n e d n o n r e s p o n s e bias a n d t h e s a m p l i n g error associated w i t h selecting a s a m p l e of t h e r e s p o n d e n t s to a previous s a m p l e limits t h e g e n e r a l i z a b i l i t y of t h e s e data. Second, a l t h o u g h t h e r e w a s no s t a t i s t i c a l difference in e i t h e r t h e n u m b e r of gasoline s t a t i o n stops or n u m b e r of visits to N e b r a s k a b e t w e e n visitors t r a v e l i n g to N e b r a s k a d e s t i n a t i o n s as c o m p a r e d to visitors w h o were t r a v e l i n g t h r o u g h t h e s t a t e e n r o u t e to a n o t h e r state, t h e r e s u l t s of t h i s s t u d y c a n n o t be generalized to a s t a t e c h a r a c t e r i z e d as a t o u r i s m d e s t i n a t i o n area. B e c a u s e of t h e large p e r c e n t a g e of t r a v e l - t h r o u g h visitors to N e b r a s k a , t h e n u m b e r of trips m e a s u r e is p r o b a b l y s u b s t a n t i a l l y h i g h e r t h a n w h a t w o u l d be expected in a d e s t i n a t i o n area. Third, t h i s s t u d y focused on potential s a m p l i n g bias, defined as m i s r e p r e s e n t a t i o n of t h e s t u d y p o p u l a t i o n r e s u l t i n g from a q u a l i t y or p r o b l e m in t h e s a m p l i n g process (Babble 1983:145); specifically, t h i s s t u d y c o n c e n t r a t e s on t h e problem of duplicate listings in t h e s a m p l i n g frame. A related, b u t different issue is t h e s a m p l i n g error a s s o c i a t e d w i t h visitor s u r v e y s , defined a s m i s r e p r e s e n t a t i o n of t h e s t u d y p o p u l a t i o n r e s u l t i n g from t h e r a n d o m error a s s o c i a t e d w i t h selecting a single probability s a m p l e (Wright 1979). T h e differences b e t w e e n t h e w e i g h t e d a n d u n w e i g h t e d v a l u e s p r e s e n t e d in t h i s p a p e r are not, in m o s t cases, large e n o u g h to completely rule out t h e possibility t h a t t h e y are a f u n c t i o n only of s a m p l i n g error. For example, u s i n g t h e f o r m u l a provided by W r i g h t (1979:113), t h e 95 274
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p e r c e n t confidence interval for t h e u n w e i g h t e d d a y s in N e b r a s k a m e a s u r e is 1.89 to 2 . 4 4 d a y s w h i c h i n c l u d e s t h e w e i g h t e d v a l u e of 1.94. Similarly, t h e 95 p e r c e n t confidence interval for t h e u n w e i g h t e d e x p e n d i t u r e s m e a s u r e is $ 1 0 8 . 9 7 to $ 1 3 4 . 5 3 , again including t h e w e i g h t e d v a l u e of $ 1 0 9 . 0 1 . However, t h e 95 p e r c e n t confidence i n t e r v a l s for t h e w e i g h t e d d a y s a n d e x p e n d i t u r e s measures, 1.71 to 2.16 d a y s a n d $ 1 0 7 . 9 9 to $ 1 1 0 . 0 3 , respectively, do not include t h e u n w e i g h t e d values. C o n s e q u e n t l y , t h e o b s e r v e d differences between the weighted and unweighted values may have b e e n a f u n c t i o n of only s a m p l i n g error. F u r t h e r r e s e a r c h with larger s a m p l e s is p r o b a b l y n e c e s s a r y to d e t e r m i n e statistically if t h e o b s e r v e d differences are t h e result of a s a m p l i n g bias, s a m p l i n g error, or a c o m b i n a t i o n thereof. Importantly, t h e potential i m p a c t of t h e participation a n d visits b i a s e s varies b y t h e t w o major t y p e s of visitor s u r v e y data. First, only a participation b i a s m a y affect visitor s u r v e y r e s u l t s c o n c e r n ing trip c h a r a c t e r i s t i c s . As w i t h previous r e s e a r c h , t h e findings from this s t u d y indicate t h a t failure to correct for this b i a s m a y result in s u b s t a n t i a l l y o v e r - e s t i m a t e d v a l u e s for m o s t trip c h a r a c teristics, p a r t i c u l a r l y length of s t a y a n d visitor e x p e n d i t u r e s . Second, b o t h a participation a n d a visits b i a s m a y affect visitor s u r v e y r e s u l t s c o n c e r n i n g visitor c h a r a c t e r i s t i c s . T h e potentially i m p o r t a n t result of this s t u d y is t h e finding t h a t t h e two b i a s e s a p p e a r to h a v e opposite effects on visitor c h a r a c t e r i s t i c data, particularly d a t a w h i c h are geographically based. Visits bias a p p e a r s to o v e r - r e p r e s e n t t h o s e visitors w h o live in s u r r o u n d i n g states. Conversely, p a r t i c i p a t i o n b i a s a p p e a r s to o v e r - r e p r e s e n t individu a l s from g r e a t e r d i s t a n c e s . T h e c o m b i n e d effect of correcting t h e d a t a for b o t h biases, at least in this study, provided r e s u l t s w h i c h w e r e very similar to t h e r a w s u r v e y data. Correcting for only one of t h e t w o b i a s e s may, however, r e s u l t in s u b s t a n t i a l l y incorrect values. Given t h e e x p e n s e a n d difficulty of o b t a i n i n g t h e n u m b e r of visits data, e s s e n t i a l l y requiring a s e c o n d s u r v e y after t h e initial study, it m a y be m o r e a p p r o p r i a t e to u s e t h e r a w s u r v e y r e s u l t s w i t h o u t correcting for either bias. F u r t h e r r e s e a r c h is recomm e n d e d to s u b s t a n t i a t e this conclusion. [] [] ACKNOWLEDGEMENTS T h e N e b r a s k a Visitor S u r v e y w a s f u n d e d b y t h e N e b r a s k a Dep a r t m e n t of E c o n o m i c D e v e l o p m e n t . T h e follow-up s a m p l i n g b i a s s u r v e y w a s f u n d e d b y t h e U n i v e r s i t y of N e b r a s k a R e s e a r c h Council. 1986 ANNALS OF TOURISM RESEARCH
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REFERENCES A b s h e r , J., a n d D. S a m d a h l 1983 F r e q u e n c y of Use a s a Source of Bias in On-Site S u r v e y s of R e c r e a t i o n i s t s . U n p u b l i s h e d p a p e r p r e s e n t e d at t h e A n n u a l S y m p o s i u m on Leisure R e s e a r c h , National R e c r e a t i o n a n d P a r k Association, K a n s a s City, Missouri, Oct. 2 - 5 . Archer, B., a n d S. S h e a 1975 L e n g t h of Stay P r o b l e m s in Tourist R e s e a r c h . J o u r n a l of Travel R e s e a r c h 12(3):8- I0. Babbie, E. R. 1983 The Practice of Social R e s e a r c h , Third Edition. Belmont, CA: W a d s w o r t h Publishing Company. Bailie, J. G.. a n d J. H. Gough 1976 An A l t e r n a t i v e Method of S u r v e y i n g I n t e r n a t i o n a l T r a v e l e r s at F r o n t i e r Points, Pilot Auto Exit Survey, S e c o n d Quarter, 1974. Ottawa: S t a t i s t i c s C a n a d a Balden A s s o c i a t e s 1978 Identifying T r a v e l e r Markets: R e s e a r c h Methodologies. C o n t r a c t r e p o r t to t h e United S t a t e s Travel Service, W a s h i n g t o n DC: U.S. D e p a r t m e n t of Commerce. Bardon, K. S., a n d D. M. Harding 1981 On-Site Q u e s t i o n n a i r e S u r v e y s in UK Leisure R e s e a r c h . I n t e r n a t i o n a l J o u r n a l of T o u r i s m M a n a g e m e n t 2(1):36 - 48. C o c h r a n , W. C. 1977 S a m p l i n g T e c h n i q u e s , Third Edition, New York: J o h n Wiley a n d Sons. Cournoyer, N. G., a n d J. K. K i n d a h l 1983 The M a s s a c h u s e t t s Travel R e s e a r c h Study. J o u r n a l of Travel R e s e a r c h 21(4):5- 11. C u m m i n g s , K. M. 1979 R a n d o m Digit Dialing: A S a m p l i n g T e c h n i q u e for T e l e p h o n e Surveys. Public Opinion Quarterly 4 3 ( 2 ) : 2 3 3 - 2 4 4 . English, M. M. 1981 The S t a t e s Agree: T h e y Love Tourists. A d v e r t i s i n g Age 52(25):$21 - $ 2 2 . Gitelson, R. J., a n d J. C r o m p t o n 1983 The P l a n n i n g Horizons a n d S o u r c e s of I n f o r m a t i o n Used by P l e a s u r e Vacationers. J o u r n a l of Travel R e s e a r c h 21 (3):2 - 7. Goodrich, J. N. 1979 R e s p o n d e n t s ' a n d N o n r e s p o n d e n t s ' Views on S t i m u l a t i n g R e s p o n s e to Mail S u r v e y s in Travel R e s e a r c h . J o u r n a i of Travel R e s e a r c h 17(3):7-11. Holland, S. M., A. J. Fedler, a n d R. B. Ditton 1983 The S p o k e s m a n Role Bias: A n o t h e r Look. U n p u b l i s h e d p a p e r p r e s e n t e d at t h e A n n u a l S y m p o s i u m on Leisure R e s e a r c h , National R e c r e a t i o n a n d P a r k Association, K a n s a s City, Missouri. Oct. 2 - 5 . Huber, D. B., a n d A. D. Rodger 1974 S a m p l e Design a n d Selection P r o c e d u r e s in Auto Exit S u r v e y s of Tourists: The Nova Scotia E x p e r i e n c e . J o u r n a l of Travel R e s e a r c h 13( 1 ) : 5 - 1 0 . Hunt, J. D., a n d G. Cadez 1978 U t a h T o u r i s m - - M o t o r Vehicle Travel: Spring, 1978 a n d Year a n d Quarterly S u m m a r i e s for S u m m e r , 1977 T h r o u g h Spring, 1978. C o n t r a c t r e p o r t to t h e U t a h Division of Travel Development, Institute of Outdoor R e c r e a t i o n a n d Tourism, U t a h State University, Logan, Utah. Kalton, G. 1983 Introduction to Survey Sampling. Beverly Hills, CA: Sage Publications, Inc. 276
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Kish, L. 1965 S u r v e y S a m p l i n g . New York: J o h n Wiley a n d Sons. K r e i s m a n , R. 1982 A l a s k a n Meltdown, T o u r i s m W a r m s U p / L o u i s i a n a ' s D r e a m of a C a m p a i g n / R h o d e I s l a n d e r s in a Proud State. A d v e r t i s i n g Age 5 3 ( 1 6 ) : 3 0 - 3 1 . Lucas, R. C. 1963 Bias in E s t i m a t i n g R e c r e a t i o n i s t ' s L e n g t h of S t a y from S a m p l e Interviews. J o u r n a l of F o r e s t r y 6 1 : 9 1 2 - 914. Lucas, R. C. 1975 Low C o m p l i a n c e R a t e s at U n m a n n e d Trail Registers. USDA F o r e s t Service R e s e a r c h Note INT-200. Ogden, UT: I n t e r m o u n t a i n F o r e s t a n d R a n g e Experiment Station. Lucas, R. C., a n d J. L. O l t m a n 1971 S u r v e y S a m p l i n g W i l d e r n e s s Visitors. J o u r n a l of Leisure R e s e a r c h 3(I):28-42. Mason, J. B. 1975 T h e Motorist T r a v e l e r a n d t h e I n t e r s t a t e Highway System: A n E x p l o r a t o r y Analysis. J o u r n a l of T r a v e l R e s e a r c h 13(3): 11 - 14. Mills, A. S., R. W. Hodgson, J. G. McNeely, a n d R. F. M a s s e 1981 A n Improved Visitor S a m p l i n g M e t h o d for Ski R e s o r t s a n d S i m i l a r Settings. J o u r n a l of Leisure R e s e a r c h 1 3 ( 3 ) : 2 1 9 - 2 3 1 . Morucei, G. 1980 T o u r i s m D a t a Processing: E l e m e n t a r y Methodology. A n n a l s of T o u r i s m Res e a r c h 7 ( 2 ) : 2 3 4 - 252. Peat, Marwick, Mitchell a n d Co. 1976 G u i d e l i n e s for D e s i g n i n g T r a v e l S u r v e y s for S t a t e w i d e T r a n s p o r t a t i o n P l a n n i n g . W a s h i n g t o n DC: F e d e r a l Highway A d m i n i s t r a t i o n . Perdue, R. R., a n d R. B. Ditton 1983 S a m p l i n g from R e g i s t r a t i o n Files: T h e P r o b l e m of Duplicate Listings. J o u r n a l of Leisure R e s e a r c h 1 5 ( 2 ) : 9 5 - 9 9 . P r i t c h a r d , G. 1982 T o u r i s m Promotion: Big B u s i n e s s for t h e States. T h e Cornell Hotel a n d Restaurant Administration Quarterly 23{2):48-57. Rao, J. N. K. 1968 S o m e N o n r e s p o n s e S a m p l i n g T h e o r y w h e n t h e F r a m e C o n t a i n s a n Unk n o w n N u m b e r of Duplications. J o u r n a l of t h e A m e r i c a n S t a t i s t i c a l A s s o c i a t i o n 63:87-90. Rovelstad, J. M., a n d S. R. Blazer 1983 R e s e a r c h a n d Strategic M a r k e t i n g in T o u r i s m : A S t a t u s Report. J o u r n a l of T r a v e l R e s e a r c h 2 2 ( 2 ) : 2 - 7. Scotler, G. W. 1981 R e s p o n s e R a t e s a t U n m a n n e d Trail Registers, W a t e r t o n L a k e s N a t i o n a l Park, Alberta, C a n a d a . J o u r n a l of Leisure R e s e a r c h 13(2): 1 0 5 - 1 1 1 . S u d m a n , S. 1976 Applied S a m p l i n g . New York: A c a d e m i c Press. U.S. D e p a r t m e n t of C o m m e r c e 1981 C r e a t i n g E c o n o m i c G r o w t h a n d J o b s T h r o u g h T r a v e l a n d T o u r i s m . W a s h i n g t o n DC: U.S. G o v e r n m e n t P r i n t i n g Office. Wagar, J. A., a n d J. F. T h a l h e i m e r 1968 C o m p u t e r P r o g r a m s for W e i g h t i n g L e n g t h of S t a y a n d O t h e r Visitor Charac_ teristics. USDA F o r e s t Service R e s e a r c h Note INT-79. Ogden, UT: I n t e r m o u n t a i n F o r e s t a n d R a n g e E x p e r i m e n t Station. 1986 ANNALS OF TOURISM RESEARCH
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Weaver, G. D. 1978 T o u r i s m U.S.A.: Volume If, Development, A s s e s s i n g Your Product a n d t h e Market, T e c h n i q u e s of Data Collection. C o n t r a c t report to t h e United S t a t e s Travel Service. D e p a r t m e n t of R e c r e a t i o n a n d P a r k A d m i n i s t r a t i o n , U n i v e r s i t y of Missouri, Columbia, Missouri. Wright, S. R. 1979 Q u a n t i t a t i v e Methods a n d Statistics: A Guide to Social R e s e a r c h . Beverly Hills, CA: Sage Publications. Wurst, C. 1955 The L e n g t h of Stay Problem in Tourist Studies. J o u r n a l of Marketing 19{4}:357- 359. S u b m i t t e d 15 D e c e m b e r 1984 Revised v e r s i o n s u b m i t t e d 2 August 1985 Accepted 18 October 1985 Refereed a n o n y m o u s l y
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ABSTRACT T h e p u r p o s e of t h i s r e s e a r c h w a s to a s s e s s t h e s a m p l i n g bias in s t a t e a u t o m o b i l e visitor s u r v e y s a s s o c i a t e d w i t h two potential sources of duplicate listings in s a m p l i n g frames obtained w h e n distributing diary questionnaires to visitors at self-selected stops w i t h i n t h e s t a t e ' s b o u n d ary. A p a r t i c i p a t i o n bias occurs w h e n e v e r t h e visitor's probability of being s u r v e y e d varies as a f u n c t i o n of h i s / h e r p a r t i c i p a t i o n in a s a m p l i n g related activity. A visits bias occurs w h e n e v e r t h e i n d i v i d u a l ' s probability of being s u r v e y e d varies as a f u n c t i o n of t h e n u m b e r of t i m e s h e / s h e visits t h e s t a t e d u r i n g t h e s u r v e y period. Keywords: methodology, survey, sampling, visitor study, tourism.
Richard Perdae (Recreation Resources Administration, North Carolina State U n i v e r s i t y , R a l e i g h NC 2 7 6 9 5 , USA) r e c e i v e d h i s P h . D i n R e c r e a t i o n R e s o u r c e s D e v e l o p m e n t f r o m T e x a s A & M U n i v e r s i t y . T h e d a t a c o l l e c t i o n for t h i s a r t i c l e w a s c o n d u c t e d w h i l e t h e a u t h o r w a s a n a s s i s t a n t p r o f e s s o r a t t h e U n i v e r s i t y of Nebraska. A n n a l s o f Tourlsm Research, Vol. 13, pp. 261-278, 1986
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SAMPLINGBIASIN VISITOR SURVEYS RI~SUME; D e f o r m a t i o n de s o n d a g e p a r double c o m p t e d a n s des enq u e t e s de visiteurs. Le propos de cette r e c h e r c h e etait d ' e v a l u e r la d e f o r m a t i o n de s o n d a g e d a n s des e n q u e t e s p a r m i des p e r s o n n e s qui v i s i t a i e n t u n e t a t en voiture. Ces e n q u e t e s p r e s e n t a i e n t d e u x s o u r c e s e v e n t u e l l e s de ded o u b l e m e n t de compte d a n s les u n i t e s de s o n d a g e qui o n t ete o b t e n u e s q u a n d on a d i s t r i b u e des q u e s t i o n n a i r e s s u r des p r o d u i t s laitiers a des a r r e t s facultatifs a l ' i n t e r i e u r de l'etat. Une d e f o r m a t i o n de p a r t i c i p a t i o n a lieu c h e q u e fois ot~ la probabilite que le visiteur p a r t i c i p e r a d a n s l ' e n q u e t e varie en f o n c t i o n de sa p a r t i c i p a t i o n ~t u n e activite qui a u n r a p p o r t avec le sujet de l'enquete. Une d e f o r m a t i o n de f r e q u e n c e de visites a lieu c h a q u e fois oh la probabilite que l'on p a r t i c i p e r a & l ' e n q u e t e varie en f o n c t i o n du n o m b r e de fois o4 l'on visite l ' e t a t p e n d a n t la d u r e e de l'enquete. Mota clef: methodologie, e n q u e t e , sondage, e t u d e s u r les visiteurs, t o u r i s m e .
INTRODUCTION Because of its potential e c o n o m i c i m p o r t a n c e , s t a t e g o v e r n m e n t s h a v e become i n c r e a s i n g l y involved in travel a n d t o u r i s m p r o g r a m s (English 1981 ; K r e i s m a n 1982; P r i t c h a r d 1982). C o n c o m i t a n t w i t h t h i s i n v o l v e m e n t is a recognized n e e d for q u a l i t y m a r k e t r e s e a r c h , c o n c e r n i n g b o t h a c t u a l a n d potential s t a t e visitors (Bardon a n d Harding, 1981; Morucci 1980; Rovelstad a n d Blazer 1983). Of t h o s e s t a t e s r e s p o n d i n g to a 1982 survey, 70 p e r c e n t reported c o n d u c t i n g t o u r i s m m a r k e t r e s e a r c h ; 65 p e r c e n t of t h e s e s t a t e s also i n d i c a t e d t h e i r use of s u c h r e s e a r c h would i n c r e a s e in t h e f u t u r e (Rovelstad a n d Blazer 1983:4). As a major c o m p o n e n t of s t a t e t o u r i s m r e s e a r c h , visitor s u r v e y s are c o n d u c t e d e i t h e r a n n u a l l y or b i e n n i a l l y to m o n i t o r t h e c h a n g e s a n d t r e n d s in a s t a t e ' s t o u r i s m i n d u s t r y . By u s i n g a relatively s m a l l sample, t h e s u r v e y s focus on i d e n t i f y i n g t h e c h a r a c t e r i s t i c s a n d b e h a v i o r s of t h e visitor population, including, a m o n g o t h e r variables, t h e i r origins, d e s t i n a t i o n s , e x p e n d i t u r e s , behaviors, a n d socio-demographic c h a r a c t e r i s t i c s (Balden Associates 1978; Peat, Marwick, Mitchell a n d Co. 1976; U.S. D e p a r t m e n t of C o m m e r c e 1981 ; W e a v e r 1978). Increasingly, due to t h e p o t e n t i a l m e a s u r e m e n t p r o b l e m s a s s o c i a t e d w i t h e i t h e r on-site i n t e r v i e w s or mail q u e s t i o n n a i r e s , t h e s e s u r v e y s are being c o n d u c t e d u s i n g a diary 262
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questionnaire methodology in w h i c h respondents are given a questionnaire to be completed during their visit in the state and ret u r n e d by mail after they leave the state (e.g., Cournoyer and Kindahl 1983). Concern over the a c c u r a c y a n d representativeness of these surveys inspires continued methodological development, particularly of sampling procedures (Bailie and Gough 1976; Cournoyer and Kindhahl 1983; Huber and Rodger 1974). F u r t h e r research is needed which carefully examines alternative sampling methodologies involving not only w h e r e to sample travelers, but also how to select survey participants (e.g., Holland, Fedler and Ditton 1983), survey administration (e.g., Mills, Hodgson, McNeely and Masse 1981), and the causes and effects of survey nonresponse (e.g., Goodrich 1979). Of particular concern to visitor survey sampling procedures is the examination a n d improvement of the available sampling frames. Importantly, although a survey sampling frame is defined as the list of sampling units composing the study population from which the sample is selected (Babble 1983), very few state visitor survey samples are selected from a n actual list of potential sampling units. Rather, the sampling frame typically comprises a quasi-list of individuals or parties visiting designated sampling points during the survey period. In most cases, due to the inability to stop visitors at ports-of-entry to the state, these sampling points are places spread throughout the state where visitors self-select to stop (e.g., interstate rest areas, gasoline service stations, attractions, etc.). For example, if a visitor survey is being conducted by contacting visitor parties at interstate rest areas during a three m o n t h period, the sampling frame comprises all visitor parties stopping at a n interstate rest area during the specified three months. The potential problems with such sampling frames can be grouped into two categories, incomplete coverage of the study population (e.g., Lucas 1975; Lucas and Oltman 1971; Scotler 1981) and duplicate listings of some sampling units (e.g., Perdue and Ditton 1983; Rao 1968). The purpose of the r e s e a r c h reported in this article is to examine two forms of duplicate listings which are potential sources of sampling bias in state visitor surveys involving the distribution of diary questionnaires to visitors at self-selected stops. First, a duplicate listing bias occurs w h e n e v e r the visitor's probability of being sampled varies as a function of his/her frequency of participation in a sampling related activity. If visitors are being contacted at a particular type of area (e.g., interstate rest areas), each visit to such a n area is a sampling frame listing. Multiple sampling frame listings 1986 ANNALS OF TOURISM RESEARCH
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m a y , therefore, exist for t h o s e people w h o visit s u c h a r e a s several t i m e s d u r i n g t h e i r s t a y in t h e s t u d y area. While it is n o t t e c h n i c a l l y correct, t h i s bias h a s b e e n t e r m e d a " l e n g t h - o f - s t a y " bias in b o t h t h e r e c r e a t i o n a n d t o u r i s m l i t e r a t u r e (Archer a n d S h e a 1975; L u c a s 1963; W u r s t 1955). It is incorrect in t h a t while t h e y are typically h i g h l y correlated, p a r t i c i p a t i o n in t h e s a m p l i n g related activity a n d t h e a m o u n t of time s p e n t in t h e s t u d y a r e a are n o t n e c e s s a r i l y t h e same. It is possible for a n individual to visit a n a r e a for a n e x t e n d e d period w i t h o u t visiting a s a m p l i n g c o n t a c t point a n d to visit a cont a c t point several t i m e s in a relatively s h o r t visit. C o n s e q u e n t l y , in t h i s paper, t h i s bias will be t e r m e d a " p a r t i c i p a t i o n b i a s . " Visitor e x p e n d i t u r e s are p e r h a p s t h e m o s t significant problem a s s o c i a t e d w i t h a p a r t i c i p a t i o n bias (Archer a n d S h e a 1975). Since participation in t h e s a m p l i n g related activity does t e n d to i n c r e a s e w i t h l e n g t h of s t a y w h i c h , in t u r n , t e n d s to be related to e x p e n d i t u r e s , t h i s bias typically r e s u l t s in a t e n d e n c y to o v e r - e s t i m a t e b o t h average visitor p a r t y e x p e n d i t u r e s a n d t h e economic i m p a c t of t o u r i s m . Second, a duplicate listing bias also occurs w h e n e v e r a visitor's s a m p l i n g probability varies as a f u n c t i o n of t h e n u m b e r of t i m e s h e / s h e visits t h e s t a t e d u r i n g t h e s u r v e y period (Absher a n d Samd a h l 1983; Babbie 1983; W a g a r a n d T h a l h e i m e r 1968). A s s u m i n g no p a r t i c i p a t i o n bias, e a c h visit by i n d i v i d u a l s m a k i n g multiple trips to a s t a t e d u r i n g t h e s u r v e y period is a s a m p l i n g f r a m e listing. C o n s e q u e n t l y , t h e probability of being s u r v e y e d for t h e s e individu a l s is s u b s t a n t i a l l y h i g h e r t h a n t h a t for t h o s e i n d i v i d u a l s w h o only m a k e one trip to t h e state. T h e i m p o r t a n c e of t h i s bias, t e r m e d in t h e r e m a i n d e r of t h i s p a p e r as a " v i s i t s b i a s , " d e p e n d s u p o n t h e u n i t of a n a l y s i s u s e d in a s s e s s i n g t h e s u r v e y r e s u l t s (Babbie 1983:146; Kish 1965). If t h e u n i t of a n a l y s i s is t h e trip, i.e., describing or c o m p a r i n g trip c h a r a c t e r i s t i c s s u c h as e x p e n d i t u r e s , l e n g t h of stay, a n d trip purpose, a visits bias is n o t relevant. However, in t h o s e s i t u a t i o n s w h e r e t h e visitor is t h e u n i t of a n a l y s i s , failure to correct for a visits bias r e s u l t s in a t e n d e n c y to over-repres e n t t h e more f r e q u e n t visitor. E x a m p l e s of s u c h a n a l y s e s include a s s e s s m e n t s of visitor c h a r a c t e r i s t i c s (e.g., origin, socio-demographics, a n d a t t i t u d e s / i m a g e s / p r e f e r e n c e s ) . As i n d i c a t e d by t h e previous citations, articles c o n c e r n i n g t h e s e b i a s e s h a v e b e e n p u b l i s h e d in b o t h t h e r e c r e a t i o n a n d t o u r i s m literature. Several a u t h o r s h a v e p u b l i s h e d empirical d a t a c o n c e r n ing t h e effects of a p a r t i c i p a t i o n bias on e s t i m a t e s of b o t h trip a n d visitor c h a r a c t e r i s t i c s (Archer a n d S h e a 1975; L u c a s 1963). However, s y s t e m a t i c a n a l y s e s of t h e potential visits bias in visitor surveys, i n c l u d i n g its e x t e n s i t y a n d effects on s u r v e y r e s u l t s b o t h indi264
1 9 8 6 ANNALS OF TOURISM R E S E A R C H
RICHARDPERDUE vidually a n d in c o m b i n a t i o n w i t h t h e effects of t h e potential p a r t i c i p a t i o n bias, h a v e n o t b e e n published. T h e p u r p o s e of t h i s r e s e a r c h w a s to c o n d u c t s u c h a n a n a l y s i s . METHODOLOGY During t h e s u m m e r of 1983 (May 2 7 t h to S e p t e m b e r 5th), d a t a were collected by d i a r y q u e s t i o n n a i r e from n o n r e s i d e n t a u t o m o b i l e visitors to Nebraska. While requiring visitors to stop at port-ofe n t r y roadblocks (e.g., H u n t a n d Cadez 1978) h a s obvious s a m p l i n g merits, t h i s s a m p l i n g procedure w a s rejected by t h e N e b r a s k a Dep a r t m e n t of Roads, b e c a u s e of t h e potential s a f e t y a n d liability problems. T h e c o m m o n a l t e r n a t i v e of s u r v e y i n g visitors at port-ofe n t r y visitor i n f o r m a t i o n c e n t e r s (e.g., Gitelson a n d C r o m p t o n 1983) w a s rejected due to t h e potential s a m p l i n g bias associated w i t h t h e inability to s u r v e y b o t h t h o s e individuals traveling on t h e i n t e r s t a t e , b u t not stopping at t h e visitor i n f o r m a t i o n c e n t e r s a n d t h o s e i n d i v i d u a l s not t r a v e l i n g on t h e i n t e r s t a t e . C o n s e q u e n t l y , visitors were c o n t a c t e d at gasoline service s t a t i o n s located t h r o u g h o u t t h e state. S a m p l i n g d a y s (102) were allocated to t w e n t y - e i g h t s u r v e y c o m m u n i t i e s on t h e basis of h i g h w a y traffic flows a n d t h e n u m b e r of h o t e l / m o t e l r o o m s in e a c h c o m m u n i t y . S a m p l i n g s t a t i o n s w i t h i n e a c h c o m m u n i t y were selected on t h e b a s i s of a t e l e p h o n e s u r v e y of local hotel/motel m a n a g e r s , essentially a s k i n g t h e m to identify t h e major s t a t i o n s for n o n r e s i d e n t visitors in t h e i r c o m m u n i t y . A m e m b e r of e a c h visitor p a r t y s t o p p i n g at t h e selected s t a t i o n s on a n a s s i g n e d s a m p l i n g d a y w a s a s k e d to participate in t h e survey. Individuals w h o agreed to participate in t h e s u r v e y were a s k e d to provide t h e i r n a m e a n d a d d r e s s on a c o n t a c t card. T h e y were t h e n given a d i a r y q u e s t i o n n a i r e to complete d u r i n g t h e i r visit. E a c h s u r v e y p a r t i c i p a n t w a s s e n t a p o s t c a r d r e m i n d e r to r e t u r n t h e q u e s t i o n n a i r e s e v e n d a y s after t h e initial contact. N o n r e s p o n d e n t s were s e n t a follow-up letter a n d a s e c o n d copy of t h e q u e s t i o n n a i r e 14 d a y s a f t e r t h e initial contact. Overall, 3782 visitor p a r t i e s were contacted, from w h i c h 3 6 3 4 individuals (96.1%) agreed to participate in t h e s u r v e y a n d 2345 individuals (62.2%) completed a n d r e t u r n e d t h e d i a r y q u e s t i o n n a i r e . Since t h e s a m p l i n g f r a m e comprised all visitor p a r t i e s e n t e r i n g gasoline service stations, potential p a r t i c i p a t i o n bias w a s m e a s u r e d by a q u e s t i o n a s k i n g respond e n t s how m a n y t i m e s t h e y stopped at gasoline s t a t i o n s d u r i n g t h e i r N e b r a s k a visit. For t h e p u r p o s e of t h i s a n a l y s i s , a r a n d o m s a m p l e of 400 d i a r y 1986 ANNALSOF TOURISMRESEARCH
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SAMPLINGBIASIN VISITOR SURVEYS q u e s t i o n n a i r e r e s p o n d e n t s w a s s u r v e y e d in a follow-up s t u d y cond u c t e d d u r i n g October, 1983. T h e s e individuals were s e n t a postc a r d q u e s t i o n n a i r e a s k i n g t h e m h o w m a n y t i m e s t h e y visited Neb r a s k a d u r i n g t h e s u m m e r (May 2 7 t h to S e p t e m b e r 5th) of 1983 a n d t h e d i s t a n c e from t h e i r h o m e to Nebraska. I m p o r t a n t l y , tourism in N e b r a s k a is d o m i n a t e d by t r a v e l - t h r o u g h visitors; 74.5 perc e n t of t h e i n d i v i d u a l s c o n t a c t e d d u r i n g t h e s u m m e r of 1983 were t r a v e l i n g t h r o u g h t h e s t a t e e n r o u t e to a n o t h e r area. A large perc e n t a g e of t h e s e visitors v a r y t h e i r r o u t e s for t h e different legs of t h e i r trip. For example, m a n y visitors travel to t h e i r d e s t i n a t i o n t h r o u g h N e b r a s k a on I n t e r s t a t e 80, b u t r e t u r n h o m e e i t h e r t h r o u g h S o u t h D a k o t a on I n t e r s t a t e 90 or t h r o u g h K a n s a s on I n t e r s t a t e 70. Similarly, m a n y visitors travel to t h e i r d e s t i n a t i o n t h r o u g h e i t h e r S o u t h D a k o t a or K a n s a s a n d r e t u r n h o m e t h r o u g h Nebraska. As a r e s u l t of t h i s b e h a v i o r a l p a t t e r n , r e s p o n d e n t s to t h e d i a r y questionn a i r e were i n s t r u c t e d to complete t h e i n s t r u m e n t only for t h a t leg of t h e i r trip on w h i c h t h e y were c o n t a c t e d . A l t h o u g h it u n d o u b t e d l y inflated t h e n u m b e r of visits m e a s u r e , r e s p o n d e n t s to t h e follow-up s u r v e y were, c o n s e q u e n t l y , a s k e d t h r e e questions: h o w m a n y t i m e s t h e y traveled to N e b r a s k a d e s t i n a t i o n s d u r i n g t h e s u m m e r of 1983, h o w m a n y t i m e s t h e y t r a v e l e d t h r o u g h t h e s t a t e e n r o u t e from t h e i r h o m e to a d e s t i n a t i o n in a n o t h e r state, a n d h o w m a n y t i m e s t h e y t r a v e l e d t h r o u g h t h e s t a t e on t h e i r w a y h o m e from a d e s t i n a t i o n . T h e total n u m b e r of visits w a s c a l c u l a t e d by a d d i n g t h e r e s p o n s e to t h e s e t h r e e questions. Using a p o s t c a r d r e m i n d e r a n d one follow-up mailing, 290 i n d i v i d u a l s completed a n d r e t u r n e d t h e p o s t c a r d q u e s t i o n n a i r e , a r e s p o n s e rate of 72.5 percent. For t h e p u r p o s e of a s s e s s i n g t h e p a r t i c i p a t i o n a n d visits s a m p l i n g biases, t h e d i a r y q u e s t i o n n a i r e d a t a for t h e s e i n d i v i d u a l s were m e r g e d w i t h t h e follow-up s u r v e y d a t a to create a final d a t a set c o m p r i s i n g 290 c a s e s a n d i n c l u d i n g both s a m p l i n g bias m e a s u r e s a n d t h e visitor a n d t h e trip c h a r a c t e r i s t i c d a t a from t h e d i a r y q u e s t i o n n a i r e . Since is w a s n o t possible, due to f i n a n c i a l c o n s t r a i n t s , to c o n d u c t t h e follow-up s u r v e y w i t h all r e s p o n d e n t s to t h e d i a r y questionnaire, t h e following r e s u l t s c a n n o t be c o n s i d e r e d r e p r e s e n t a t i v e of t h e N e b r a s k a visitor population. As s h o w n in Table 1, t h e r e were potential differences in t h e visitor a n d trip c h a r a c t e r i s t i c s of t h e follow-up s u r v e y r e s p o n d e n t s as c o m p a r e d to all visitor s u r v e y res p o n d e n t s , p r o b a b l y r e s u l t i n g from a c o m b i n a t i o n of b o t h s a m p l i n g error a n d n o n r e s p o n s e s a m p l i n g bias. T h e d a t a were, however, c o n s i d e r e d acceptable to e x a m i n e t h e potential p a r t i c i p a t i o n a n d visits s a m p l i n g bias a s s o c i a t e d w i t h t h e d i a r y q u e s t i o n n a i r e m e t h odology. 266
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C o m p a r i s o n o f Nebraska Visitor S u r v e y a n d F o l l o w - u p Survey Results Variable
N e b r a s k a Visitor S u r v e y
Follow-up S u r v e y
42.4
44.9
46.0% 54.0
48.4% 51.6
30.8% 30.8 38.4
31.8% 29.3 38.9
19.8% 80.2
20.8% 79.2
18.3% 17.8 25.3 19.1 19.6
16.7 % 21.2 25.7 17.0 19.4
38.6%
37.2%
19.4
19.0
8.6
10.3
Visitor Characteristics 1. Age" 2. Sex Male Female 3. E d u c a t i o n High school or less Some college College g r a d u a t e 4. E x p o s u r e to N e b r a s k a T o u r i s m Ads Yes No
Trip Characteristics 1. Trip P l a n n i n g Period Less t h a n two w e e k s Two to four w e e k s One to t h r e e m o n t h s F o u r to six m o n t h s Over six m o n t h s 2. P r i m a r y Trip P u r p o s e Visiting family a n d friends Recreation and sightseeing Business and convention All o t h e r p u r p o s e s 3. Days in N e b r a s k a " 4. E x p e n d i t u r e s a 5. A t t r a c t i o n Visits a 6. Highway Rest A r e a Visits a
33.4 2.26 122.50 0.40 2.32
33.4 2.17 121.60 0.36 2.35
a mean values
RESULTS The frequency distributions for the sampling measures are pres e n t e d i n T a b l e 2. T h e f o l l o w - u p s u r v e y r e s p o n d e n t s a v e r a g e d 2 . 2 4 trips to or through Nebraska during the summer of 1983 and, on the 1986 ANNALS OF TOURISM RESEARCH
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Table 2 F r e q u e n c y Distribution o f Gasoline Station Stops and Visits to Nebraska Visits to Nebraska Frequency
Visitors freq. pct.
One Two Three Four Five Six or more Total
109 106 41 18 5 11 290
Visits freq. pct.
37.6 109 36.6 212 14.1 123 6.2 72 1.7 25 3.8 109 100.0 650 ~ = 2.24
16.8 32.6 18.9 11.1 3.8 16.8 100.0
Gasoline Station Stops Visitors freq. pct. 81 123 45 16 7 4 276
Gas Stops freq. pct.
29.3 81 44.6 246 16.3 135 5.8 64 2.5 35 1.5 26 100.0 587 ~ = 2.13
13.8 41.6 23.0 10.9 4.4 4.4 100.0
s u r v e y e d trip, 2.13 stops at gasoline service s t a t i o n s . Over 60 perc e n t of t h e r e s p o n d e n t s m a d e more t h a n one trip to t h e state. Similarly, a l m o s t 71 p e r c e n t of t h e r e s p o n d e n t s m a d e more t h a n one stop at a gasoline service s t a t i o n . Of t h e reported 650 N e b r a s k a visits, 50.6 p e r c e n t were m a d e by f r e q u e n t visitors (-> 3 trips to t h e state). F r e q u e n t visitors, however, r e p r e s e n t e d only 25.8 p e r c e n t of t h e r e s p o n d e n t s . Of t h e reported 587 stops a t gasoline service stations, 260 (44.3%) were by f r e q u e n t s t o p p e r s (-> 3 stops), 26.1 perc e n t of t h e r e s p o n d e n t s . S u b s t a n t i a l potential exists, therefore, for duplicate listings in t h e visitor s u r v e y s a m p l i n g frame. Given t h i s p o t e n t i a l duplicate listing problem, a n a l y s e s were n e x t c o n d u c t e d of t h e r e l a t i o n s h i p b e t w e e n t h e selected visitor c h a r a c t e r i s t i c s a n d trips to N e b r a s k a a n d n u m b e r of gasoline station stops v a r i a b l e s (Table 3). Trips to N e b r a s k a varied w i t h age, origin ( m e a s u r e d on t h e follow-up p o s t c a r d q u e s t i o n n a i r e as t h e d i s t a n c e from t h e i n d i v i d u a l ' s h o m e to Nebraska), a n d w h e t h e r or n o t t h e individual h a d s e e n or h e a r d N e b r a s k a t o u r i s m advertisem e n t s prior to leaving home. F u r t h e r a n a l y s i s identified significant i n t e r r e l a t i o n s h i p s b e t w e e n t h e s e variables w h i c h e m p h a s i z e d t h e i m p o r t a n c e of t h e d i s t a n c e from h o m e to N e b r a s k a m e a s u r e . Specifically, N e b r a s k a ' s t o u r i s m p r o m o t i o n a l efforts are focused on a geographical m a r k e t w i t h i n 500 miles of t h e s t a t e ' s border. Conseq u e n t l y , as d i s t a n c e from t h e s t a t e i n c r e a s e d , e x p o s u r e to N e b r a s k a t o u r i s m a d v e r t i s e m e n t s decreased. Similarly, as d i s t a n c e from Neb r a s k a increased, visitor age also decreased. T h e n u m b e r of gasoline s t a t i o n stops w a s significantly related only to t h e e d u c a t i o n a l 268
1 9 8 6 ANNALS OF TOURISM R E S E A R C H
RICHARD PERDUE Table 3
Visits to Nebraska and Gasoline Station Stops by Visitor Characteristics Visits to Nebraska Visitor Characteristic
Mean
Age Sex Male Female Education High school Some college College graduate Distance from Home to Nebraska Exposure to Nebraska Tourism Ads Yes No
Statistic
Gasoline Station Stops Mean
r = .07 F = 0.25
r = .14" F = 0.73 2.15 2.08
2.28 2. I 0
F = 4.06"
F = 1.90 2.32 2.18 1.89
2.46 2.18 1.96 r = --. 14"
r=
F = 8.02 b 2.87 2.09
Statistic
.11
F---- 1.55
1.95 2.15
a significant at alpha ---.05 b significant at alpha <-.01
level of t h e r e s p o n d e n t s . I n d i v i d u a l s w i t h h i g h e r l e v e l s of e d u c a t i o n t e n d e d to s t o p a t g a s o l i n e s t a t i o n s l e s s f r e q u e n t l y t h a n t h o s e w i t h less education. S i m i l a r a n a l y s e s w e r e c o n d u c t e d to e x a m i n e t h e r e l a t i o n s h i p bet w e e n s e l e c t e d t r i p c h a r a c t e r i s t i c s a n d t h e n u m b e r of g a s o l i n e s t a t i o n s t o p s m e a s u r e ( T a b l e 4). A s e x p e c t e d , t h e n u m b e r of g a s o l i n e s t a t i o n s t o p s w a s p o s i t i v e l y r e l a t e d to b o t h t h e n u m b e r of d a y s a n d t h e a m o u n t o f m o n e y s p e n t in t h e s t a t e . S i n c e b o t h g a s o l i n e s e r v i c e s t a t i o n s a n d h i g h w a y rest a r e a s serve as r e s t r o o m s a n d s o u r c e s of i n f o r m a t i o n for t o u r i s t s , t h e s t r e n g t h o f t h e r e l a t i o n s h i p b e t w e e n those two variables was not expected. Further analysis indicated t h a t r e s t a r e a v i s i t s w e r e s i g n i f i c a n t l y r e l a t e d to t h e n u m b e r of d a y s s p e n t in t h e s t a t e . H e n c e , t h e o b s e r v e d r e s t a r e a v i s i t s to g a s o l i n e station stops relationship was probably spurious. Similarly, the o b s e r v e d r e l a t i o n s h i p t h a t t h e l e n g t h of t h e t r i p p l a n n i n g p e r i o d d e c r e a s e d w i t h t h e n u m b e r of g a s s t o p s w a s n o t e x p e c t e d . A s s u m i n g t h a t t h e t r i p p l a n n i n g p e r i o d w o u l d l e n g t h e n w i t h t h e n u m b e r of d a y s in N e b r a s k a , it w a s a l s o e x p e c t e d to l e n g t h e n w i t h t h e n u m b e r 1986 ANNALS OF TOURISM RESEARCH
269
S A M P L I N G BIAS IN V I S I T O R S U R V E Y S
Table 4
Gasoline Station Stops by Trip Characteristics Gasoline Station Stops Trip Characteristic Trip P l a n n i n g Period Less t h a n two w e e k s Two to four w e e k s One to three m o n t h s Four to six m o n t h s Over six m o n t h s P r i m a r y Trip Purpose Visiting family a n d friends Recreation a n d Sightseeing Business a n d convention All other p u r p o s e s Days in N e b r a s k a Expenditures Attraction Visits Highway Rest Area Visits
Mean
Statistic F = 2.57"
2.40 2.33 2.00 1.98 1.84 F = 0.60 2.31 2.13 2.09 1.98 r---r= r= r=
.19 b .32 b .03 .26 b
a s i g n i f i c a n t a t a l p h a --- . 0 5 b s i g n i f i c a n t a t a l p h a -< .001
of g a s o l i n e s t a t i o n s t o p s . T h e r e w a s , h o w e v e r , n o r e l a t i o n s h i p b e t w e e n t h e l e n g t h of t h e t r i p p l a n n i n g p e r i o d a n d t h e n u m b e r of d a y s s p e n t in N e b r a s k a , p r o b a b l y b e c a u s e of t h e l a r g e p e r c e n t a g e of N e b r a s k a v i s i t o r s t r a v e l i n g t h r o u g h t h e s t a t e e n r o u t e to a n o t h e r d e s t i n a t i o n . T h e n u m b e r of d a y s s p e n t in N e b r a s k a w a s n o t r e l a t e d to t h e o v e r a l l l e n g t h of t h e v i s i t o r ' s t r i p . Correcting an existing data base for a sampling bias resulting f r o m d u p l i c a t e l i s t i n g s in t h e s a m p l i n g f r a m e r e q u i r e s t h e d e v e l o p m e n t of a w e i g h t i n g f a c t o r w h i c h is i n v e r s e l y p r o p o r t i o n a l to t h e visitor's selection probability (Cochran 1977; Cummings 1979; K a l t o n 1 9 8 3 ; S u d m a n 1976). If t h e v i s i t o r ' s s e l e c t i o n p r o b a b i l i t y Pl is k n o w n , a w e i g h t i n g c o e f f i c i e n t of k / p i c a n b e u s e d , w h e r e k is a c o n v e n i e n t c o n s t a n t . T h e o b v i o u s c h o i c e of k = 1, s o t h a t t h e w e i g h t wl ---- 1/p~, is p a r t i c u l a r l y u s e f u l w h e n e s t i m a t e s of p o p u l a t i o n t o t a l s a r e n e e d e d . F o r e x a m p l e , w i t h v i s i t o r e x p e n d i t u r e s el, Ew~e, p r o v i d e s t h e t o t a l t o u r i s m e x p e n d i t u r e f o r t h e v i s i t o r p o p u l a t i o n . H o w e v e r , s i n c e t h e p~ in v i s i t o r s t u d i e s a r e f r e q u e n t l y v e r y s m a l l v a l u e s , k ---- 1 t y p i c a l l y r e s u l t s in n o n - i n t e g e r w e i g h t s w h i c h r e q u i r e s e v e r a l d e c i m a l p l a c e s a n d a r e difficult to u s e w i t h s o m e 270
1986 ANNALS OF TOURISM RESEARCH
RICHARD PERDUE
c o m p u t e r s t a t i s t i c a l packages. C o n s e q u e n t l y , a n a l t e r n a t i v e value of k is f r e q u e n t l y u s e d in visitor s t u d i e s to simplify t h e w e i g h t s a n d t h e i r application. A l t h o u g h e s t i m a t e s of p o p u l a t i o n p a r a m e t e r s s u c h as m e a n , v a r i a n c e , a n d p e r c e n t a g e d i s t r i b u t i o n s will still be correct w i t h v a l u e s o t h e r t h a n k ---- 1, s u m m e d v a l u e s s u c h as Ew~e~ m u s t be divided by k to derive e s t i m a t e s of p o p u l a t i o n totals. Since in m o s t visitor s t u d i e s p~ is u n k n o w n b u t a s s u m e d to be equal for all visitors, a w e i g h t i n g coefficient o t h e r t h a n k/pl is needed. Given t h a t differential s a m p l i n g probabilities r e s u l t in visitor s u r v e y s from duplicate listings in t h e s a m p l i n g f r a m e a s s o c i a t e d w i t h t h e visitor's p a r t i c i p a t i o n in a s a m p l i n g related activity 11a n d n u m b e r of visits to t h e s t a t e d u r i n g t h e s u r v e y period v~, k/l~ a n d k/v~ provide w e i g h t s w h i c h c a n be u s e d to correct visitor s u r v e y d a t a for t h e two s a m p l i n g f r a m e problems, respectively. For t h o s e variables i n f l u e n c e d by duplicate listings a s s o c i a t e d w i t h both p a r t i c i p a t i o n a n d visits, w~--(k/ll) + (k/v~) provides t h e a p p r o p r i a t e w e i g h t i n g c o e f f c i e n t (Wagar a n d T h a l h e i m e r , 1968). Again, e s t i m a t e s of p o p u l a t i o n p a r a m e t e r s s u c h as m e a n , v a r i a n c e , a n d p e r c e n t a g e d i s t r i b u t i o n s will be correct w i t h t h e s e w e i g h t i n g coefficients. Population totals are e s t i m a t e d by correcting for t h e e s t i m a t e d n u m b e r of visitors to t h e s t a t e d u r i n g t h e s u r v e y period. For example, a n e s t i m a t e of total visitor e x p e n d i t u r e s would be: N
~ wlel i~l
N
W
where wl -- p a r t i c i p a t i o n w e i g h t i n g coeffcient, el = visitor e x p e n d i t u r e , N---- s a m p l e size, a n d V ----e s t i m a t e d n u m b e r of s t a t e visitors d u r i n g t h e s u r v e y period. Using t h e s e f o r m u l a s , w e i g h t e d v a l u e s for t h e selected trip a n d visitor c h a r a c t e r i s t i c s of t h e N e b r a s k a Visitor S u r v e y were calculated for t h e follow-up s u r v e y r e s p o n d e n t s a n d are p r e s e n t e d , along w i t h t h e u n w e i g h t e d values, in Tables 5 a n d 6. C o m p a r i s o n of t h e w e i g h t e d a n d u n w e i g h t e d visitor c h a r a c t e r i s tic v a l u e s revealed potential differences, p a r t i c u l a r l y on t h e dist a n c e from h o m e to N e b r a s k a a n d e x p o s u r e to N e b r a s k a t o u r i s m a d v e r t i s e m e n t s m e a s u r e s {Table 5). However, t h e effects of t h e participation a n d visits b i a s e s t e n d e d to offset e a c h o t h e r in s u c h a w a y t h a t t h e c o m b i n a t i o n of t h e two w e i g h t i n g factors r e s u l t e d in v a l u e s 1986 ANNALS OF TOURISM RESEARCH
271
S A M P L I N G BIAS IN V I S I T O R S U R V E Y S
Table 5
Weighted and Unweighted Visitor Characteristic Values Values Weighted by
Visitor Characteristic Agea Sex Male Female Education High school or less Some college College graduate Distance from home to Nebraska a Exposure to Nebraska Tourism Advertisements Yes No
Unweighted Values
Nebraska Visits
Gasoline Station Stops
Gasoline Station Stops and Nebraska Visits
44.9
44.1
44.5
44.2
48.4% 51.6
47.4% 52.6
47.3% 52.7
47.6% 52.4
31.8% 29.3 38.9 565.7
29.7% 28.5 41.8 591.9
29.7% 27.4 42.9 537.0
28.9% 28.4 42.7 567.3
20.8% 79.2
18.4% 81.6
23.4% 76.6
21.3% 78.7
"mean values
o n l y slightly d i f f e r e n t f r o m t h e u n w e i g h t e d val ues. C o n s e q u e n t l y , w hile r e p l i c a t i o n s w i t h a d d i t i o n a l d a t a b a s e s a r e n e c e s s a r y , it app e a r s t h a t t h e visitor c h a r a c t e r i s t i c s of t h e follow-up s u r v e y d a t a base were not biased w h e n comparing the unweighted values with t h o s e w e i g h t e d to c o r r e c t for b o t h t h e p a r t i c i p a t i o n a n d visits b i as es . H o w e v e r , c o r r e c t i o n for o n l y o n e of t h e t w o p o t e n t i a l duplic a t e listing b i a s e s m a y r e s u l t in s u b s t a n t i a l l y i n c o r r e c t v a l u e s , p a r t i c u l a r l y o n m e a s u r e s a s s o c i a t e d w i t h vi s it or origin a n d e x p o s u r e to s t a t e p r o m o t i o n a l efforts. C o m p a r i s o n of t h e u n w e i g h t e d a n d w e i g h t e d v a l u e s in T a b l e 6 reflects t h e p o t e n t i a l i m p o r t a n c e of c o r r e c t i n g vi si t or s u r v e y t ri p c h a r a c t e r i s t i c d a t a for a possible p a r t i c i p a t i o n bias. T h e w e i g h t e d v a l u e s for d a y s in N e b r a s k a , e x p e n d i t u r e s , a n d n u m b e r of r e s t a r e a visits a p p e a r s u b s t a n t i v e l y s m a l l e r t h a n t h e u n w e i g h t e d val ues, p o t e n t i a l l y i n d i c a t i n g t h e bi a s in r e p o r t i n g u n w e i g h t e d data. T h e u n w e i g h t e d d a y s a n d e x p e n d i t u r e s v a l u e s a r e 11.8 a n d 11.5 perc e n t g r e a t e r t h a n t h e w e i g h t e d val ues , r e s p e c t i v e l y . Similarly, 272
1986 ANNALS OF TOURISM RESEARCH
RICHARDPERDUE Table 6
Weighted and Unweighted Trip Characteristic Values Trip Characteristic Trip Planning Period Less than two weeks Two to four weeks One to three months Four to six months Over six months Primary Trip Purpose Visiting family and friends Recreation and sightseeing Business and convention All other purposes Days in Nebraska a Expenditures" Attraction Visits a Highway Rest Area Visits a
Unweighted Values
Values Weighted by Number of Gasoline Station Stops
16.7% 21.2 25.7 17.0 19.4
18.9% 21.7 26.5 15.9 17.0
37.2% 19.0 10.3 33.4 2.17 121.60 0.36 2.35
37.0% 19.9 10.0 32.3
1.94 109.01 0.33 2.20
a mean values
w h e n c o r r e c t e d for t h e n u m b e r of g a s o l i n e s t a t i o n stops, visitor t ri p p l a n n i n g a p p e a r s s h o r t e r t e r m t h a n i n d i c a t e d by t h e u n w e i g h t e d data. CONCLUSIONS S a m p l i n g v is i t or s to a large g e o g r a p h i c a l a r e a w i t h r e l a t i v e l y o p e n a c c e s s f r o m a v a r i e t y of p o i n t s is a n e x t r e m e l y c o m p l e x m e t h odological p r o b l e m . In all b u t a few cas e s , in s t a t e visitor s u r v e y s t h i s p r o b l e m is f u r t h e r m a g n i f i e d by t h e i n a b i l i t y to stop vi si t ors at p o r t s - o f - e n t r y , d u e to s a f e t y a n d liability c o n c e r n s . T h e p u r p o s e of t h i s r e s e a r c h w a s to e x a m i n e t h e p o t e n t i a l s a m p l i n g bi as assoc i a t e d w i t h two s o u r c e s of d u p l i c a t e listings in t h e s a m p l i n g f r a m e o b t a i n e d w h e n d i s t r i b u t i n g d i a r y q u e s t i o n n a i r e s to vi si t ors at selfselected stops within the state's boundary. A potential participat i o n b ias e x i s t s w h e n e v e r t h e v i s i t o r ' s p r o b a b i l i t y of b e i n g s a m p l e d v a r i e s as a f u n c t i o n of h o w f r e q u e n t l y h e / s h e p a r t i c i p a t e s in a specific activity. A p o t e n t i a l visits bi as e x i s t s w h e n e v e r t h e individu a l ' s p r o b a b i l i t y of b e i n g s a m p l e d v a r i e s as a f u n c t i o n of h o w freq u e n t l y h e / s h e visits t h e s t a t e d u r i n g t h e s u r v e y period. 1986 ANNALSOF TOURISM RESEARCH
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SAMPLINGBIASIN VISITOR SURVEYS Prior to m a k i n g a n y c o n c l u s i o n s c o n c e r n i n g t h e r e s u l t s of t h i s s t u d y , it is i m p o r t a n t to m a k e t h r e e points. First, in order to a s s e s s t h e effects of a visits bias, it is n e c e s s a r y to c o n d u c t a follow-up s u r v e y w i t h r e s p o n d e n t s to t h e d i a r y q u e s t i o n n a i r e . To d e t e r m i n e t h e n u m b e r of visits to t h e s t a t e d u r i n g t h e entire visitor s u r v e y period, t h i s follow-up s u r v e y m u s t be c o n d u c t e d following t h e e n d of t h a t period. It is u n r e a s o n a b l e to expect a complete r e s p o n s e to t h i s follow-up survey. C o n s e q u e n t l y , t h e potential n o n r e s p o n s e bias in t h e follow-up s u r v e y r e s u l t s m a y limit t h e i r value in correcting t h e visitor s u r v e y data. A l t h o u g h several a u t h o r s h a v e c o m m e n t e d on t h e potential for a visits bias in visitor s u r v e y r e s e a r c h , previous d a t a c o n c e r n i n g its effects h a v e n o t b e e n published, p r o b a b l y bec a u s e of t h e e x p e n s e a n d difficulty a s s o c i a t e d w i t h t h i s follow-up survey. A p r i m a r y p u r p o s e of t h i s methodological note is to provide s u c h data. Due to t h e a s s o c i a t e d expense, however, it w a s necess a r y to c o n d u c t t h e follow-up s u r v e y w i t h only a s m a l l s a m p l e of t h e 1983 N e b r a s k a Visitor S u r v e y r e s p o n d e n t s . C o n s e q u e n t l y , alt h o u g h t h e r a w d a t a v a l u e s for t h e follow-up s u r v e y were s i m i l a r to t h o s e for t h e overall visitor survey, t h e c o m b i n a t i o n of t h e previously m e n t i o n e d n o n r e s p o n s e bias a n d t h e s a m p l i n g error associated w i t h selecting a s a m p l e of t h e r e s p o n d e n t s to a previous s a m p l e limits t h e g e n e r a l i z a b i l i t y of t h e s e data. Second, a l t h o u g h t h e r e w a s no s t a t i s t i c a l difference in e i t h e r t h e n u m b e r of gasoline s t a t i o n stops or n u m b e r of visits to N e b r a s k a b e t w e e n visitors t r a v e l i n g to N e b r a s k a d e s t i n a t i o n s as c o m p a r e d to visitors w h o were t r a v e l i n g t h r o u g h t h e s t a t e e n r o u t e to a n o t h e r state, t h e r e s u l t s of t h i s s t u d y c a n n o t be generalized to a s t a t e c h a r a c t e r i z e d as a t o u r i s m d e s t i n a t i o n area. B e c a u s e of t h e large p e r c e n t a g e of t r a v e l - t h r o u g h visitors to N e b r a s k a , t h e n u m b e r of trips m e a s u r e is p r o b a b l y s u b s t a n t i a l l y h i g h e r t h a n w h a t w o u l d be expected in a d e s t i n a t i o n area. Third, t h i s s t u d y focused on potential s a m p l i n g bias, defined as m i s r e p r e s e n t a t i o n of t h e s t u d y p o p u l a t i o n r e s u l t i n g from a q u a l i t y or p r o b l e m in t h e s a m p l i n g process (Babble 1983:145); specifically, t h i s s t u d y c o n c e n t r a t e s on t h e problem of duplicate listings in t h e s a m p l i n g frame. A related, b u t different issue is t h e s a m p l i n g error a s s o c i a t e d w i t h visitor s u r v e y s , defined a s m i s r e p r e s e n t a t i o n of t h e s t u d y p o p u l a t i o n r e s u l t i n g from t h e r a n d o m error a s s o c i a t e d w i t h selecting a single probability s a m p l e (Wright 1979). T h e differences b e t w e e n t h e w e i g h t e d a n d u n w e i g h t e d v a l u e s p r e s e n t e d in t h i s p a p e r are not, in m o s t cases, large e n o u g h to completely rule out t h e possibility t h a t t h e y are a f u n c t i o n only of s a m p l i n g error. For example, u s i n g t h e f o r m u l a provided by W r i g h t (1979:113), t h e 95 274
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p e r c e n t confidence interval for t h e u n w e i g h t e d d a y s in N e b r a s k a m e a s u r e is 1.89 to 2 . 4 4 d a y s w h i c h i n c l u d e s t h e w e i g h t e d v a l u e of 1.94. Similarly, t h e 95 p e r c e n t confidence interval for t h e u n w e i g h t e d e x p e n d i t u r e s m e a s u r e is $ 1 0 8 . 9 7 to $ 1 3 4 . 5 3 , again including t h e w e i g h t e d v a l u e of $ 1 0 9 . 0 1 . However, t h e 95 p e r c e n t confidence i n t e r v a l s for t h e w e i g h t e d d a y s a n d e x p e n d i t u r e s measures, 1.71 to 2.16 d a y s a n d $ 1 0 7 . 9 9 to $ 1 1 0 . 0 3 , respectively, do not include t h e u n w e i g h t e d values. C o n s e q u e n t l y , t h e o b s e r v e d differences between the weighted and unweighted values may have b e e n a f u n c t i o n of only s a m p l i n g error. F u r t h e r r e s e a r c h with larger s a m p l e s is p r o b a b l y n e c e s s a r y to d e t e r m i n e statistically if t h e o b s e r v e d differences are t h e result of a s a m p l i n g bias, s a m p l i n g error, or a c o m b i n a t i o n thereof. Importantly, t h e potential i m p a c t of t h e participation a n d visits b i a s e s varies b y t h e t w o major t y p e s of visitor s u r v e y data. First, only a participation b i a s m a y affect visitor s u r v e y r e s u l t s c o n c e r n ing trip c h a r a c t e r i s t i c s . As w i t h previous r e s e a r c h , t h e findings from this s t u d y indicate t h a t failure to correct for this b i a s m a y result in s u b s t a n t i a l l y o v e r - e s t i m a t e d v a l u e s for m o s t trip c h a r a c teristics, p a r t i c u l a r l y length of s t a y a n d visitor e x p e n d i t u r e s . Second, b o t h a participation a n d a visits b i a s m a y affect visitor s u r v e y r e s u l t s c o n c e r n i n g visitor c h a r a c t e r i s t i c s . T h e potentially i m p o r t a n t result of this s t u d y is t h e finding t h a t t h e two b i a s e s a p p e a r to h a v e opposite effects on visitor c h a r a c t e r i s t i c data, particularly d a t a w h i c h are geographically based. Visits bias a p p e a r s to o v e r - r e p r e s e n t t h o s e visitors w h o live in s u r r o u n d i n g states. Conversely, p a r t i c i p a t i o n b i a s a p p e a r s to o v e r - r e p r e s e n t individu a l s from g r e a t e r d i s t a n c e s . T h e c o m b i n e d effect of correcting t h e d a t a for b o t h biases, at least in this study, provided r e s u l t s w h i c h w e r e very similar to t h e r a w s u r v e y data. Correcting for only one of t h e t w o b i a s e s may, however, r e s u l t in s u b s t a n t i a l l y incorrect values. Given t h e e x p e n s e a n d difficulty of o b t a i n i n g t h e n u m b e r of visits data, e s s e n t i a l l y requiring a s e c o n d s u r v e y after t h e initial study, it m a y be m o r e a p p r o p r i a t e to u s e t h e r a w s u r v e y r e s u l t s w i t h o u t correcting for either bias. F u r t h e r r e s e a r c h is recomm e n d e d to s u b s t a n t i a t e this conclusion. [] [] ACKNOWLEDGEMENTS T h e N e b r a s k a Visitor S u r v e y w a s f u n d e d b y t h e N e b r a s k a Dep a r t m e n t of E c o n o m i c D e v e l o p m e n t . T h e follow-up s a m p l i n g b i a s s u r v e y w a s f u n d e d b y t h e U n i v e r s i t y of N e b r a s k a R e s e a r c h Council. 1986 ANNALS OF TOURISM RESEARCH
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