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Willingness to pay for antihypertensive care: Evidence from a staff-model HMO
Pergamon
So('. Sci, Med. Vol. 44, No. 12, pp. 1911-1917, 1997
Plh
S0277-9536(96)00300-0
~ 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0277-9536/97 $17,00 + 0.00
W I L L I N G N E S S TO P A Y F O R A N T I H Y P E R T E N S I V E CARE: EVIDENCE FROM A STAFF-MODEL HMO S C O T T D. R A M S E Y ? ¢* S E A N D. S U L L I V A N , =~ B R U C E M. PSATY ''3'4'S and D O N A L D L. P A T R I C K 4 'Department of Medicine, University of Washington, Seattle, WA~ U.S.A., 2Department of Pharmacy, University of Washington, Seattle, WA, U.S.A., ~Cardiovascular Health Research Unit, University of Washington, Seattle, WA, U.S.A., 4Department of Health Services, University of Washington, Seattle, WA, U.S.A. and ~Department of Epidemiology, University of Washington, Seattle, WA, U.S.A. Abstract Willingness to pay (WTP) has been used in Sweden to evaluate the value of antihypertensive therapy. The Swedish studies indicate that hypertensive patients are willing to pay between $107 and $120 per month for their therapy. We conducted a similar study in a population of hypertensive patients in a large, staff-model, managed care organization (Group Health Cooperative of Puget Sound). Participants returned a postal survey containing a WTP question with 10 "bids" ranging from $25 to $250. Respondents recorded whether they would accept or reject each bid at the stated dollar value. Demographic information such as age, income, and perceived health status was also collected. Results were analyzed with ordinary least squares regression, controlling for the demographic data. The estimated parameters were significant and indicated an adjusted mean WTP of $93 per month for antihypertensive therapy. The close similarity of the Swedish and U.S. results appears to support the use of WTP as a measure of health state preferences. ~ 1997 Elsevier Science Ltd
Key words--willingness to pay, hypertension, HMO
INTRODUCTION
Willingness to pay (WTP) has received renewed attention as a method of measuring the benefits of health care interventions (Johannesson et al., 1992; Johannesson and Jonsson, 1991; O'Brien and Viramontes, 1994). While researchers in several countries have performed W T P studies for a variety of health care interventions, international comparisons of W T P for therapy for particular medical conditions have not yet been performed. It is unclear whether the W T P results from one country would apply to another, since medical care practices, reimbursement systems, incomes, and tastes and preferences for health care will differ from country to country. Nevertheless, an international comparison of W T P for a specific condition would provide valuable information regarding the relevance of the results in one country for another country, particularly if both countries share similar demographic characteristics. In addition, the similarity or difference in results from repeated studies on separate populations would provide evidence in support of or against the robustness of the W T P method as a measure for eliciting health state preferences. We designed a W T P study of willingness to pay for antihypertensive therapy that was similar to pre*Author for correspondence. Mail Stop 358853, 169 N. Canal Street, Suite 300, Seattle, WA 98103, U.S.A. SSM 44 12 P
vious studies in hypertensive patients performed by Johannesson et al. in Sweden (Johannesson et al., 1991, 1993). Our objectives were to: (1) evaluate W T P as a measure of health state preferences in patients with hypertension; and (2) compare and contrast our results with the Swedish studies. The international comparison of results is discussed in the context of the health care delivery systems for both countries, and the reported strengths and limitations of the W T P method for eliciting health state preferences. METHODS
Setting and sample The willingness to pay survey was part of a larger mailed survey of quality of life related to antihypertensive therapy, conducted at the Group Health Cooperative of Puget Sound (GHC), a large, staffmodel H M O located in Washington State. The study was approved by the H u m a n Subjects Committee of G H C and of the University of Washington. We identified 400 subjects who had treated hypertension and were 50-79 years old. Because this survey was part of a larger survey of health-related quality of life related to hypertension and its sequelae, we oversampled subjects with myocardial infarction or stroke so that the prevalence of subjects with these conditions were 19% and 7.5%, respectively (the expected prevalence were 10% and 4%, respectively). The sample was
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designed to have approximately equal numbers of men and women. In May of 1994, questionnaires were mailed to the sample of 400 patients treated for hypertension at primary care clinics. Nonrespondents received a second questionnaire two weeks later; subjects who did not return the second questionnaire received reminder phonecalls. As part of this pilot study, subjects were randomized to receive either $3 (n = 194) or no money (n = 206) with the initial copy of mailed questionnaire. Survey instrument design
The WTP question contained 10 separate WTP "bids", ranging from $25 to $250, in $25 increments. For each bid, individuals were asked to respond by checking one of the following: yes, definitely; yes, probably; no, probably not; no, definitely not; don't know. The range of dollar values was comparable to that of Johannesson et al. The question was designed to simulate a bidding game where respondents would be less likely to accept the bid as the dollar values rose in relation to their monthly income (see Appendix A for exact question wording). Respondents were also asked to rate their perceived health with and without treatment for hypertension on a 10 cm category rating scale (CRS) that has been widely used in previous health care studies (Freyd, 1923; Scott and Huskisson, 1977). After examining the CRS scale, which ranged from 0 (death) to 100 (perfect health), respondents wrote in numbers by the scale that they felt most closely matched their perceived health with and without treatment. The difference (CRS) represented the patient's estimated health benefit obtained through treatment. Socioeconomic questions such as household income, education, age, sex, and marital status were also included in the questionnaire. Patients were also asked to rate their current general health on a 1 5 scale (1 = excellent, 5 = poor). Analysis
The survey results were analyzed using ordinary least squares (OLS) regression, with the largest dollar value of the "yes, definitely" or "yes, probably" response to WTP bids as the dependent variable. Independent variables in the initial regression included income, CRS, age, sex, education, marital status, prior history of stroke or myocardial infarction, and perceived level of present health. The data were analyzed on a personal computer using S P S S for Windows ~ (SPSS). To test whether the certainty of response affected the results, an additional regression was run using only the highest "yes, definitely" bid as the dependent variable. Adjusted mean WTP values were then computed from each regression model. Finally, to account for the small clustering of WTP results at the $250 bid (see next section), we transformed the WTP to its natural
logarithm, reanalyzed with OLS, calculated expected WTP, then retransformed back to a dollar WTP value using a smearing estimator that has been used previously (Duan, 1983).
RESULTS The final response rates were 85% for the $3 arm and 76% for the $0 arm (P-value = 0.025). A number of respondents in the $3 arm returned their payment with the questionnaire ($61 of the $582). The overall response rate was 80%. Among respondents, 30% answered the WTP questions in a way that was considered uninterpretable (for example, by writing "inappropriate question" or "I'd pay any amount" or checking "don't know" for every bid in the answer section). The average age of respondents with interpretable WTP responses was 70 years (SD 7.95). Respondents were 50% male, 90% Caucasian, 5% black, and 2.5% Asian or Pacific islander. Over 45% had attended some college, and 21% had completed their college degrees. Mean household income was $32,050 (median $30,000). The WTP values varied from approximately 1% to 10% of respondents' median monthly income. Maximum WTP values showed a bimodal distribution, with 51% accepting either the $25 and $50 bids, and 9% accepting the $250 bid. The mean and median unadjusted WTP responses were $94.67 (SD 76) and $50, respectively. The average monthly income of the $25 and $50 respondents was $2371, compared to $3783 for the $250 respondents (Table 1). Table 1 shows the association between levels of WTP and related demographic and health status measures. The mean WTP levels for patients without a prior history of myocardial infarction or stroke did not differ significantly from the WTP values of those who had experienced one of these conditions in the past (P = 0.211), although the stroke patients' mean bid ($117) was slightly higher than either the controls ($93) or the myocardial infarction patients ($94). Their WTP was not strongly associated with current perceived general health status (Pearson r = 0.23). The responses to the WTP bids were analyzed with ordinary least squares (OLS) regressions (Table 2). Initially, the highest "yes, definitely" or "yes, probably" response was coded as the maximum WTP value, Respondents were willing to pay more for their antihypertensive therapy if they perceived their treatments to be beneficial or if they had a higher level of income. Current perceived health status, age, gender, and education were not statistically associated with respondents' WTP. Adding these variables did not improve the overall explanatory power of the regression model. Variables representing prior history of myocardial
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Table 1. Summary statistics (mean values listed for each column, SD in parentheses) Maximum willingness to pay $50 (n = 57) $75-150 (n = 64)
$25 (n = 58) Age
71 (7) $26,519 ($18,528) 2.31 (0.57) 16.62 (17.59) 034 0.09 (0.28) 0.17 (0.38) 0.72 3 (1) 0.1 (0.31)
Income Education" CRS ~' Male gender History stroke History myocardial infarction Proportion married Present health 1 = excellent, 5 = poor Current diabetes
68 (7) $30,459 ($17,741 ) 2.39 (0.65) 21.27 (18.6) 0.63 0.02 (0.13) 0.3 (0.46) 0.79 3 (1) 0.14 (0.35)
69 (7) $38,307 ($27,265) 2.64 (0.521 23.31 (18.27) 0.53 0.08 (0.27) 0. I 1 (0.31) 0.63 3 (1) 0.14 (0.35)
$175-250 (n = 46) 67 (9) $44,286 ($25,977) 2.48 (0.62) 28.69 (18.31) 0.7 0.13 (0.34) 0.24 (0.43) 0.85 3 (1) 0.22 (0.42)
~Education was coded as follows: 1 for primary education, 2 for secondary education (completed high school), 3 for university education. bThe perceived value of hypertensive therapy, measured as the difference in perceived health with and without treatment on a category rating scale (CRS). Note that the CRS measured 15 cm for the Swedish study and 10 cm for the U.S. study.
i n f a r c t i o n a n d p r i o r h i s t o r y o f s t r o k e were positively b u t n o n s i g n i f i c a n t l y a s s o c i a t e d w i t h W T P
m o n t h l y W T P w a s $90.79 ( T a b l e 2, W T P - 2 ) . Finally, t r a n s f o r m i n g W T P to its n a t u r a l l o g a r i t h m ,
( T a b l e 2, W T P - 1 ) . W h e n t h e m y o c a r d i a l i n f a r c t i o n a n d s t r o k e v a r i a b l e s a r e r e p l a c e d w i t h a single vari-
reanalyzing the data using OLS, calculating standardized expected WTP, and retransformation
able r e p r e s e n t i n g a h i s t o r y o f either incident, this v a r i a b l e w a s also positively b u t n o n s i g n i f i c a n t l y ass o c i a t e d w i t h W T P . S u b s t i t u t i o n o f this v a r i a b l e for
using the smearing estimator produced an expected W T P o f $90.15 [Table 2, I n ( W T P ) ] .
t h e s e p a r a t e v a r i a b l e s d i d i m p r o v e t h e significance o f t h e a g e variable. U s i n g t h e coefficients f r o m t h e full r e g r e s s i o n a n d t h e c o m b i n e d s t r o k e a n d m y o cardial
infarction
variable,
the
adjusted
mean
R e s t r i c t i n g t h e a n a l y s i s to o n l y t h e h i g h e s t " y e s , definitely" r e s p o n s e d i d n o t i m p r o v e t h e m o d e l . In t h e b e s t fitting m o d e l ( u s i n g o n l y i n c o m e a n d C R S ) , t h e coefficients for t h e v a r i a b l e s were less significant, and although the adjusted R2 improved
Table 2. Ordinary least squares regression coefficients (t-ratios within parentheses) Variable
WTP- 1
WTP*2
Ln(WTP)
Intercept
94.63 (1.325) 0.9378 7.71E-04~' (2.846) -0.742 (-0.851) 12.981 (1.083) 1.346 (0.165) -2.675 (-0.367) 16.146 (1.103) 21.581 (0.988)
118.08b (2.246) 1.0408 6.84E-04b (2.972) -1.079 ~ I-1.752) 15.98 (1.581 ) -0.264 (-0.037) -2.409 (-0.041)
4.213b (7.642) 0.011 b 7.14E-06b (2.954) -0.009 (-1.452) 0.204a (1.922) 0.058 (0.777) -0.033 (-0.517)
9.111 (0.818) 225 5.78 0.130 $90.79
0.073 (0.620) 225 6.22 0.14 $90.15c
CRS Income Age Gender Education Perceived health Previous myocardial infarction Previous stroke Previous myocardial infarction or stroke n F Adj. R2 Mean adjusted WTP
225 3.80 0.118 $94.85
~Coefficient statistically significant for ct = 0.10, bCoefficient statistically significant for • = 0.05, CRetransformed to dollar value using smearing estimator ~
=
1.302.
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slightly (from 0.127 to 0.139), the F statistic fell from 13.49 to 11.0. The adjusted mean W T P value using this approach was $28.53. Our method of using respondents' highest "yes, probably" bid is somewhat conservative, since WTP will presumably be somewhere in the range between the highest bid that is accepted and the next bid. To account for this issue, we reanalyzed the data using a W T P value that was the midpoint between the respondent's highest accepted bid and the next highest bid. Using this approach and the combined stroke and myocardial infarction variable, the adjusted W T P is $103. Analysis for trends among respondents who gave uninterpretable W T P responses was performed with logistic regression. The dependent variable was an uninterpretable response (Y = 1, N = 0). Independent variables included age, gender, education, and income. Giving an uninterpretable response was significantly associated with increasing age (P = 0.048), but not significantly associated with the other explanatory variables.
DISCUSSION We conducted a W T P survey of patients receiving care for hypertension in a large, staff-model H M O . The adjusted mean W T P of the respondents was $91. Income and perceived value of treatment were positively and significantly associated with a higher maximum WTP. Our study is similar to a study of W T P for hypertension care conducted by Johannesson et al. in Sweden in 1991 (Johannesson et al., 1993). The WTP question in the Swedish study contained a single bid, and subjects were asked whether or not they agreed to accept a single "offer to pay" bid for their hypertensive care. Subjects were randomly divided into groups, and each group was presented with a different offer to pay. Offers to pay varied from approximately $10 to $245 per month. The question allowed for different degrees of certainty in the response. The mean W T P was about $130.
Although the study populations are from different developed Western countries with different health care delivery systems, the results are quite similar (Table 3). Controlling for the income of the Swedish cohort, the adjusted W T P values were very close. Several demographic characteristics may help to explain the similarity of the results. Both populations are primarily white and middle class. The Swedish national health system and Group Health Cooperative provide relatively unlimited access to primary care services, with small co-payments for services and medications. On the other hand, the United States and Sweden are different cultures that may have different tastes, preferences, and attitudes towards health and medical care. In addition, the approach to managing hypertension may be different for each country. Limitations
Our study was part of a larger mailed survey of quality of life related to antihypertensive therapy. Since we were not able to perform an intervieweradministered W T P survey, and we hoped to avoid the problems that can arise from using an openended response format (U.S. Dept of Commerce, 1993), we designed our W T P question as a series of incrementally increasing bids. Our expectation was that as the bid values increased, respondents would change their answers from "yes, definitely" to "no, definitely not". The design of this question apparently confused many respondents, since several answered " d o n ' t know" for every bid. The likelihood of giving an uninterpretable response was significantly associated with increasing age. In addition, nonresponse bias may have affected the results. It is possible that the distribution of the potential WTP responses for those who didn't return the survey or failed to answer the W T P question in an interpretable way may be different from the distribution of the respondents' values. Although the WTP question was reviewed by clinicians that are familiar with the costs and benefits of antihypertensive therapies, it was not pretested with patients. The high proportion of uninterpretable responses suggests that cognitive survey methods may be needed to craft questions
Table 3. Comparison of U.S. and Swedish willingnessto pay surveys (SDs in parentheses) Variable Sex (% male) Age Monthly household income (1994 U.S. dollars) Educationb CRS~ Willingness to pay
U,S. 50 70.1 (7.95) 2671 (1975) 2.43 (0.60) 2.13 (1.89) $91 ($93 income-adjusted)d
Sweden [5] 46 59.6 (11.5) $2724a (1443) 1.33 (0.68) 5.08 (3.82) $117
~Updated to 1994 dollars using the Swedish consumer price index and current exchange rate. bCoded l for primary education, 2 for secondary education (completed high school), 3 for university education. CThe perceived value of hypertensive therapy, measured as the difference in perceived health with and without treatment on a category rating scale (CRS). Note that the CRS measured 15 cm for the Swedish study and l0 cm for the U.S. study. aThe adjusted U.S. WTP value uses the higher income of the Swedish group to facilitate comparison between the populations.
Willingness to pay for antihypertensive care that are acceptable and understandable to many respondents. Another option that may be preferred when feasible is administering WTP questions using trained interviewers. The lack of association between respondents' WTP and their prior history of stroke, myocardial infarction, and perceived present general health status was unexpected, although the coefficients for the individual events and the combined variable had the expected signs. Preferences for medical therapies are expected to show variation as a function of individual health status. Stroke and myocardial infarction increase the risk of future events and therefore increase the absolute risk reduction of hypertension treatment. It is likely that these coefficients did not achieve significance because the number of patients with these conditions was comparatively small. It is possible, however, that most of the myocardial infarction patients had little change in their overall health perception after the event. Some advocates of the WTP method have suggested that such surveys should be done by faceto-face interviews using a referendum format to minimize many of the problems that frequently arise when non-interviewer-administered techniques are used (U.S. Dept of Commerce, 1993; Mitchell and Carson, 1989). Because interviewer-administered surveys are time-consuming, resource-intensive, and subject to variabilities in interviewer training and experience, researchers have sought to develop alternative methods to obtain valid and reliable WTP estimates. Johannesson et al.'s binary bid WTP question reduces the likelihood of encountering starting point bias and range bias (Johannesson et al., 1993). In an earlier study of WTP for antihypertensive therapy by their group (Johannesson et al., 1991), an open-ended question format was used to avoid range and starting point bias; however, the open-ended question led to a large number of nonresponse and protest answers. Our bid format avoided open-ended responses, but had the disadvantage of being truncated at a maximum value. Since 9% of our respondents indicated that they would be willing to pay $250 per month for antihypertensive care, it is possible that a subset of this group would have paid more if higher values had been available as responses. Although starting point bias was not an issue in the more recent study Swedish study (Johannesson et al., 1993), it may have a problem with range bias, since the maximum possible response was set by the investigators. In that study. 24% responded "yes" to the maximum binary bid. In practice, the theoretical advantages of the binary bid method must be weighed against the programmatic difficulties of creating a multiple-format questionnaire, particularly when second or third mailings are used to improve response rates. In addition, the binary method employed by
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Johannesson et al. will generally require a larger sample size than is typically needed for health status surveys. Investigators may be unwilling or unable to sample more patients simply to accommodate the WTP question, unless WTP is the primary question that drives the study design. Another limitation of the study is that it surveys a relatively homogeneous population in a single insurance plan. There is evidence that preferences for specific health interventions may vary among different health insurance plans (Goddeeris, 1984; Grazier et al., 1986). If true, then it is possible that WTP for hypertension care might be different for individuals who are not insured by a staff-model HMO. From a policy perspective, this suggests that investigators in the United States may have to control for insurance status or benefit package in WTP evaluations for health care interventions, Multicollinearity between the age, education, and income variables may explain some of the problems with the explanatory power of the regression models. This issue was also noted in the Swedish study. A possible bias with WTP estimation is that the range of possible responses is capped at a level beyond which respondents are not able to indicate increasingly higher WTP values. Aggregation of responses at the highest value may lead to an underestimation of the true mean WTP. To adjust for this problem, Donaldson et al. (1995) have proposed a two-stage model to account for possible right censoring. We specified a two-stage model, using logit to estimate the instrument (first stage) and linear regression of the natural log values of WTP to estimate the second stage. The censoring bias coefficient estimated from the first stage was not statistically significant when used in the second stage, thus indicating either that our instrument was not correctly specified or that we did not have a censoring bias. In either case, the estimated mean WTP was generally the same. While this approach may be useful in cases where a large number of responses are at the ceiling value, in our study with proportionately few responses at the highest WTP category, it did not improve estimation. Willingness to pay has practical appeal because: (1) benefits are measured in monetary terms, thus permitting "stand alone" evaluation of a program, since net value is determined as the difference between benefits and costs; and (2) indirect and intangible benefits of treatment, which are typically difficult to quantify in cost-effectiveness studies, are incorporated into the monetary valuations of respondents. The WTP method is also theoretically appealing for use in cost-effectiveness analysis, because it avoids the problems that arise when costs and benefits are measured in different units. Despite these advantages, the WTP method suffers from a number of methodological and practical
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limitations (U.S. Dept of Commerce, 1993; Mitchell and Carson, 1989). First, to make an informed and accurate choice, respondents need near-perfect information regarding the health state, including its natural history and the costs and effectiveness of alternative therapies. In the case of this study, the important information that patients may lack is information about the risks of myocardial infarction and stroke with and without hypertension treatment. If this information is not clearly conveyed by physicians to patients, estimating the value of treating high blood pressure becomes extremely difficult, since the effects of the disease (stroke, myocardial infarction) are distant and catastrophic. In principle, it may be possible to give accurate treatmentassociated risk reduction information to patients, although estimating the present value of such information on a patient-specific basis is an admittedly difficult test. Second, even with adequate information, several factors may compel respondents to report inaccurately their WTP. For example, subjects who worry that their responses could affect their out-of-pocket medication capes might give falsely low W T P values. Finally, many argue that assigning a monetary value to human life and health is impossible or unethical. In response to these criticisms, Gafni has suggested that WTP questions should be framed in the context of hypothetical insurance purchasing, given that a service would be available if needed (Gafni, 1991). Despite these concerns, the findings from our study and the Swedish survey support the validity of W T P as a measure of benefits for hypertensive care. In addition to their similarity in adjusted mean response values, both studies are a relatively accurate estimate of the annual cost of hypertensive care. Published reports estimate the annual direct medical care cost of hypertension therapy in the United States to range from $411 to $1425 (1994 dollars), depending on the type of drug used for treatment (Hilleman et al., 1994; Odell and Gregory, 1995). In Sweden, the costs of hypertension treatment are lower ($400 500) (Johannesson et al., 1993). An unanswered question is how closely the costs of antihypertensive therapy match the present value of the expected future savings resulting from treatment-reduced rates of coronary disease and stroke (including indirect and intangible treatment benefits). We believe that these results support the notion that W T P is a valid and reliable method for measuring patient's perceived benefits from hypertensive care. Much of the criticism of W T P has grown out of studies of environmental legislation, which involve tasks of tremendous abstraction, such as the knowledge of the true environmental impact of the Exxon Valdez oil spill, for situations with which most individuals have no first-hand experience (U.S. Dept of Commerce, 1993). Patients know their own health. This study suggests that
they also know the consequences of their hypertension on their future health, and the value of the treatments they receive for this condition. REFERENCES
Donaldson, C., Shackley, P., Abdalla, M. and Miedzybrodzka, Z. (1995) Willingness to pay for antenatal carrier screening for cystic fibrosis. Journal of Health Economics 4(6), 439-452. Duan, N. (1983) Smearing estimate: a nonparametric retransformation method. Journal of the American Statistical Association 78, 605 610. Freyd, M. (1923) The graphic rating scale. Journal of Educational Psyehology 14, 83-102. Gafni, A. (1991) Willingness-to-pay as a measure of benefits: relevant questions in the context of public decisionmaking about health care programs. Medical Care 29, 246-1252. Goddeeris, J. H. (1984) Insurance and incentives for innovation in medical care. Southern Economic Journal 51, 530-539. Grazier, K., Richardson, D., Martin, D. and Diehr, P. (1986) Factors affecting choice of health care plans. Health Services Research 20, 659 682. Hilleman, D. E., Mohiuddin, S. M. and Lucas, B. D. (1994) Cost-minimization analysis of initial antihypertensive therapy in patients with mild-to-moderate essential diastolic hypertension. Clinical Therapeutics 16, 88-102. Johannesson, M. and Jonsson, B. (1991) Economics evaluation in health care: is there a role for cost-benefit analysis? Health Policy 20, 309. Johannesson, M., Jonsson, B, and Borgquist, L. (1991) Willingness to pay for antihypertensive therapy--results of a Swedish pilot study. Journal of Health Economics 10, 461 474. Johannesson, M., Johansson, P.-O. and Jonsson, B. (1992) Economic evaluation of drug therapy. Pharmacoeconomics 1,325 337. Johannesson, M., Johansson, P -O., Kristr6m, B. and Gerdtham, U. G. (1993) Willingness to pay for antihypertensive therapy--further results. Journal of Health Economies 12, 95 108. Mitchell, R. C. and Carson, R. T. (1989) Using Surveys to Value Public Goods." The Contingent Valuation Method.
Resources for the Future, Washington, DC. O'Brien, B. and Viramontes, J. L. (1994) Willingness to pay: a valid and reliable measure of health state preference? Medical Decision Making 14, 289-297. Odell, T. W. and Gregory, M. C. (1995) Cost of hypertension treatment. Journal of General Internal Medicine 10, 686-688. Scott, J. and Huskisson, E. C. (1977) Measurement of functional capacity with visual analogue scales. Rheumatology Rehabilitation 16, 257-259. U.S. Department of Commerce, National Oceanic and Atmospheric Administration (1993) Report of the NOAA panel on contingent valuation. Federal Register 58(10), 4601-4614.
APPENDIX A
This question concerns how much money you would be willing to pay for your high blood pressure care, including office visits and medicine. Assume for the moment that your insurance plan did not cover the costs of visits and medicine for high blood pressure. Assume that each of the dollar values listed below might represent your separate
Willingness to p a y for a n t i h y p e r t e n s i v e c a r e
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m o n t h l y bill f o r h i g h b l o o d p r e s s u r e care. F o r e a c h m o n t h l y cost, please circle the a n s w e r i n d i c a t i n g w h e t h e r o r n o t y o u w o u l d be willing to p a y t h a t a m o u n t o f m o n e y o u t o f y o u r o w n p o c k e t to c o n t i n u e y o u r t r e a t m e n t f o r h i g h b l o o d pressure. (Circle y o u r a n s w e r s . )
a. $25 a month b. $50 a month c. $75 a month d. $100 a month e, $125 a month f. $150 a month g. $175 a month h. $200 a month i. $225 a month j. $250 a month
Yes, definitely 1 1 1 1 1 1 1 1 I I
Yes, probably 2 2 2 2 2 2 2 2 2 2
No, probably not 3 3 3 3 3 3 3 3 3 3
No, definitely not 4 4 4 4 4 4 4 4 4 4
Don't know 5 5 5 5 5 5 5 5 5 5
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~ 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0277-9536/97 $17,00 + 0.00
W I L L I N G N E S S TO P A Y F O R A N T I H Y P E R T E N S I V E CARE: EVIDENCE FROM A STAFF-MODEL HMO S C O T T D. R A M S E Y ? ¢* S E A N D. S U L L I V A N , =~ B R U C E M. PSATY ''3'4'S and D O N A L D L. P A T R I C K 4 'Department of Medicine, University of Washington, Seattle, WA~ U.S.A., 2Department of Pharmacy, University of Washington, Seattle, WA, U.S.A., ~Cardiovascular Health Research Unit, University of Washington, Seattle, WA, U.S.A., 4Department of Health Services, University of Washington, Seattle, WA, U.S.A. and ~Department of Epidemiology, University of Washington, Seattle, WA, U.S.A. Abstract Willingness to pay (WTP) has been used in Sweden to evaluate the value of antihypertensive therapy. The Swedish studies indicate that hypertensive patients are willing to pay between $107 and $120 per month for their therapy. We conducted a similar study in a population of hypertensive patients in a large, staff-model, managed care organization (Group Health Cooperative of Puget Sound). Participants returned a postal survey containing a WTP question with 10 "bids" ranging from $25 to $250. Respondents recorded whether they would accept or reject each bid at the stated dollar value. Demographic information such as age, income, and perceived health status was also collected. Results were analyzed with ordinary least squares regression, controlling for the demographic data. The estimated parameters were significant and indicated an adjusted mean WTP of $93 per month for antihypertensive therapy. The close similarity of the Swedish and U.S. results appears to support the use of WTP as a measure of health state preferences. ~ 1997 Elsevier Science Ltd
Key words--willingness to pay, hypertension, HMO
INTRODUCTION
Willingness to pay (WTP) has received renewed attention as a method of measuring the benefits of health care interventions (Johannesson et al., 1992; Johannesson and Jonsson, 1991; O'Brien and Viramontes, 1994). While researchers in several countries have performed W T P studies for a variety of health care interventions, international comparisons of W T P for therapy for particular medical conditions have not yet been performed. It is unclear whether the W T P results from one country would apply to another, since medical care practices, reimbursement systems, incomes, and tastes and preferences for health care will differ from country to country. Nevertheless, an international comparison of W T P for a specific condition would provide valuable information regarding the relevance of the results in one country for another country, particularly if both countries share similar demographic characteristics. In addition, the similarity or difference in results from repeated studies on separate populations would provide evidence in support of or against the robustness of the W T P method as a measure for eliciting health state preferences. We designed a W T P study of willingness to pay for antihypertensive therapy that was similar to pre*Author for correspondence. Mail Stop 358853, 169 N. Canal Street, Suite 300, Seattle, WA 98103, U.S.A. SSM 44 12 P
vious studies in hypertensive patients performed by Johannesson et al. in Sweden (Johannesson et al., 1991, 1993). Our objectives were to: (1) evaluate W T P as a measure of health state preferences in patients with hypertension; and (2) compare and contrast our results with the Swedish studies. The international comparison of results is discussed in the context of the health care delivery systems for both countries, and the reported strengths and limitations of the W T P method for eliciting health state preferences. METHODS
Setting and sample The willingness to pay survey was part of a larger mailed survey of quality of life related to antihypertensive therapy, conducted at the Group Health Cooperative of Puget Sound (GHC), a large, staffmodel H M O located in Washington State. The study was approved by the H u m a n Subjects Committee of G H C and of the University of Washington. We identified 400 subjects who had treated hypertension and were 50-79 years old. Because this survey was part of a larger survey of health-related quality of life related to hypertension and its sequelae, we oversampled subjects with myocardial infarction or stroke so that the prevalence of subjects with these conditions were 19% and 7.5%, respectively (the expected prevalence were 10% and 4%, respectively). The sample was
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designed to have approximately equal numbers of men and women. In May of 1994, questionnaires were mailed to the sample of 400 patients treated for hypertension at primary care clinics. Nonrespondents received a second questionnaire two weeks later; subjects who did not return the second questionnaire received reminder phonecalls. As part of this pilot study, subjects were randomized to receive either $3 (n = 194) or no money (n = 206) with the initial copy of mailed questionnaire. Survey instrument design
The WTP question contained 10 separate WTP "bids", ranging from $25 to $250, in $25 increments. For each bid, individuals were asked to respond by checking one of the following: yes, definitely; yes, probably; no, probably not; no, definitely not; don't know. The range of dollar values was comparable to that of Johannesson et al. The question was designed to simulate a bidding game where respondents would be less likely to accept the bid as the dollar values rose in relation to their monthly income (see Appendix A for exact question wording). Respondents were also asked to rate their perceived health with and without treatment for hypertension on a 10 cm category rating scale (CRS) that has been widely used in previous health care studies (Freyd, 1923; Scott and Huskisson, 1977). After examining the CRS scale, which ranged from 0 (death) to 100 (perfect health), respondents wrote in numbers by the scale that they felt most closely matched their perceived health with and without treatment. The difference (CRS) represented the patient's estimated health benefit obtained through treatment. Socioeconomic questions such as household income, education, age, sex, and marital status were also included in the questionnaire. Patients were also asked to rate their current general health on a 1 5 scale (1 = excellent, 5 = poor). Analysis
The survey results were analyzed using ordinary least squares (OLS) regression, with the largest dollar value of the "yes, definitely" or "yes, probably" response to WTP bids as the dependent variable. Independent variables in the initial regression included income, CRS, age, sex, education, marital status, prior history of stroke or myocardial infarction, and perceived level of present health. The data were analyzed on a personal computer using S P S S for Windows ~ (SPSS). To test whether the certainty of response affected the results, an additional regression was run using only the highest "yes, definitely" bid as the dependent variable. Adjusted mean WTP values were then computed from each regression model. Finally, to account for the small clustering of WTP results at the $250 bid (see next section), we transformed the WTP to its natural
logarithm, reanalyzed with OLS, calculated expected WTP, then retransformed back to a dollar WTP value using a smearing estimator that has been used previously (Duan, 1983).
RESULTS The final response rates were 85% for the $3 arm and 76% for the $0 arm (P-value = 0.025). A number of respondents in the $3 arm returned their payment with the questionnaire ($61 of the $582). The overall response rate was 80%. Among respondents, 30% answered the WTP questions in a way that was considered uninterpretable (for example, by writing "inappropriate question" or "I'd pay any amount" or checking "don't know" for every bid in the answer section). The average age of respondents with interpretable WTP responses was 70 years (SD 7.95). Respondents were 50% male, 90% Caucasian, 5% black, and 2.5% Asian or Pacific islander. Over 45% had attended some college, and 21% had completed their college degrees. Mean household income was $32,050 (median $30,000). The WTP values varied from approximately 1% to 10% of respondents' median monthly income. Maximum WTP values showed a bimodal distribution, with 51% accepting either the $25 and $50 bids, and 9% accepting the $250 bid. The mean and median unadjusted WTP responses were $94.67 (SD 76) and $50, respectively. The average monthly income of the $25 and $50 respondents was $2371, compared to $3783 for the $250 respondents (Table 1). Table 1 shows the association between levels of WTP and related demographic and health status measures. The mean WTP levels for patients without a prior history of myocardial infarction or stroke did not differ significantly from the WTP values of those who had experienced one of these conditions in the past (P = 0.211), although the stroke patients' mean bid ($117) was slightly higher than either the controls ($93) or the myocardial infarction patients ($94). Their WTP was not strongly associated with current perceived general health status (Pearson r = 0.23). The responses to the WTP bids were analyzed with ordinary least squares (OLS) regressions (Table 2). Initially, the highest "yes, definitely" or "yes, probably" response was coded as the maximum WTP value, Respondents were willing to pay more for their antihypertensive therapy if they perceived their treatments to be beneficial or if they had a higher level of income. Current perceived health status, age, gender, and education were not statistically associated with respondents' WTP. Adding these variables did not improve the overall explanatory power of the regression model. Variables representing prior history of myocardial
Willingness to pay for antihypertensive care
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Table 1. Summary statistics (mean values listed for each column, SD in parentheses) Maximum willingness to pay $50 (n = 57) $75-150 (n = 64)
$25 (n = 58) Age
71 (7) $26,519 ($18,528) 2.31 (0.57) 16.62 (17.59) 034 0.09 (0.28) 0.17 (0.38) 0.72 3 (1) 0.1 (0.31)
Income Education" CRS ~' Male gender History stroke History myocardial infarction Proportion married Present health 1 = excellent, 5 = poor Current diabetes
68 (7) $30,459 ($17,741 ) 2.39 (0.65) 21.27 (18.6) 0.63 0.02 (0.13) 0.3 (0.46) 0.79 3 (1) 0.14 (0.35)
69 (7) $38,307 ($27,265) 2.64 (0.521 23.31 (18.27) 0.53 0.08 (0.27) 0. I 1 (0.31) 0.63 3 (1) 0.14 (0.35)
$175-250 (n = 46) 67 (9) $44,286 ($25,977) 2.48 (0.62) 28.69 (18.31) 0.7 0.13 (0.34) 0.24 (0.43) 0.85 3 (1) 0.22 (0.42)
~Education was coded as follows: 1 for primary education, 2 for secondary education (completed high school), 3 for university education. bThe perceived value of hypertensive therapy, measured as the difference in perceived health with and without treatment on a category rating scale (CRS). Note that the CRS measured 15 cm for the Swedish study and 10 cm for the U.S. study.
i n f a r c t i o n a n d p r i o r h i s t o r y o f s t r o k e were positively b u t n o n s i g n i f i c a n t l y a s s o c i a t e d w i t h W T P
m o n t h l y W T P w a s $90.79 ( T a b l e 2, W T P - 2 ) . Finally, t r a n s f o r m i n g W T P to its n a t u r a l l o g a r i t h m ,
( T a b l e 2, W T P - 1 ) . W h e n t h e m y o c a r d i a l i n f a r c t i o n a n d s t r o k e v a r i a b l e s a r e r e p l a c e d w i t h a single vari-
reanalyzing the data using OLS, calculating standardized expected WTP, and retransformation
able r e p r e s e n t i n g a h i s t o r y o f either incident, this v a r i a b l e w a s also positively b u t n o n s i g n i f i c a n t l y ass o c i a t e d w i t h W T P . S u b s t i t u t i o n o f this v a r i a b l e for
using the smearing estimator produced an expected W T P o f $90.15 [Table 2, I n ( W T P ) ] .
t h e s e p a r a t e v a r i a b l e s d i d i m p r o v e t h e significance o f t h e a g e variable. U s i n g t h e coefficients f r o m t h e full r e g r e s s i o n a n d t h e c o m b i n e d s t r o k e a n d m y o cardial
infarction
variable,
the
adjusted
mean
R e s t r i c t i n g t h e a n a l y s i s to o n l y t h e h i g h e s t " y e s , definitely" r e s p o n s e d i d n o t i m p r o v e t h e m o d e l . In t h e b e s t fitting m o d e l ( u s i n g o n l y i n c o m e a n d C R S ) , t h e coefficients for t h e v a r i a b l e s were less significant, and although the adjusted R2 improved
Table 2. Ordinary least squares regression coefficients (t-ratios within parentheses) Variable
WTP- 1
WTP*2
Ln(WTP)
Intercept
94.63 (1.325) 0.9378 7.71E-04~' (2.846) -0.742 (-0.851) 12.981 (1.083) 1.346 (0.165) -2.675 (-0.367) 16.146 (1.103) 21.581 (0.988)
118.08b (2.246) 1.0408 6.84E-04b (2.972) -1.079 ~ I-1.752) 15.98 (1.581 ) -0.264 (-0.037) -2.409 (-0.041)
4.213b (7.642) 0.011 b 7.14E-06b (2.954) -0.009 (-1.452) 0.204a (1.922) 0.058 (0.777) -0.033 (-0.517)
9.111 (0.818) 225 5.78 0.130 $90.79
0.073 (0.620) 225 6.22 0.14 $90.15c
CRS Income Age Gender Education Perceived health Previous myocardial infarction Previous stroke Previous myocardial infarction or stroke n F Adj. R2 Mean adjusted WTP
225 3.80 0.118 $94.85
~Coefficient statistically significant for ct = 0.10, bCoefficient statistically significant for • = 0.05, CRetransformed to dollar value using smearing estimator ~
=
1.302.
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slightly (from 0.127 to 0.139), the F statistic fell from 13.49 to 11.0. The adjusted mean W T P value using this approach was $28.53. Our method of using respondents' highest "yes, probably" bid is somewhat conservative, since WTP will presumably be somewhere in the range between the highest bid that is accepted and the next bid. To account for this issue, we reanalyzed the data using a W T P value that was the midpoint between the respondent's highest accepted bid and the next highest bid. Using this approach and the combined stroke and myocardial infarction variable, the adjusted W T P is $103. Analysis for trends among respondents who gave uninterpretable W T P responses was performed with logistic regression. The dependent variable was an uninterpretable response (Y = 1, N = 0). Independent variables included age, gender, education, and income. Giving an uninterpretable response was significantly associated with increasing age (P = 0.048), but not significantly associated with the other explanatory variables.
DISCUSSION We conducted a W T P survey of patients receiving care for hypertension in a large, staff-model H M O . The adjusted mean W T P of the respondents was $91. Income and perceived value of treatment were positively and significantly associated with a higher maximum WTP. Our study is similar to a study of W T P for hypertension care conducted by Johannesson et al. in Sweden in 1991 (Johannesson et al., 1993). The WTP question in the Swedish study contained a single bid, and subjects were asked whether or not they agreed to accept a single "offer to pay" bid for their hypertensive care. Subjects were randomly divided into groups, and each group was presented with a different offer to pay. Offers to pay varied from approximately $10 to $245 per month. The question allowed for different degrees of certainty in the response. The mean W T P was about $130.
Although the study populations are from different developed Western countries with different health care delivery systems, the results are quite similar (Table 3). Controlling for the income of the Swedish cohort, the adjusted W T P values were very close. Several demographic characteristics may help to explain the similarity of the results. Both populations are primarily white and middle class. The Swedish national health system and Group Health Cooperative provide relatively unlimited access to primary care services, with small co-payments for services and medications. On the other hand, the United States and Sweden are different cultures that may have different tastes, preferences, and attitudes towards health and medical care. In addition, the approach to managing hypertension may be different for each country. Limitations
Our study was part of a larger mailed survey of quality of life related to antihypertensive therapy. Since we were not able to perform an intervieweradministered W T P survey, and we hoped to avoid the problems that can arise from using an openended response format (U.S. Dept of Commerce, 1993), we designed our W T P question as a series of incrementally increasing bids. Our expectation was that as the bid values increased, respondents would change their answers from "yes, definitely" to "no, definitely not". The design of this question apparently confused many respondents, since several answered " d o n ' t know" for every bid. The likelihood of giving an uninterpretable response was significantly associated with increasing age. In addition, nonresponse bias may have affected the results. It is possible that the distribution of the potential WTP responses for those who didn't return the survey or failed to answer the W T P question in an interpretable way may be different from the distribution of the respondents' values. Although the WTP question was reviewed by clinicians that are familiar with the costs and benefits of antihypertensive therapies, it was not pretested with patients. The high proportion of uninterpretable responses suggests that cognitive survey methods may be needed to craft questions
Table 3. Comparison of U.S. and Swedish willingnessto pay surveys (SDs in parentheses) Variable Sex (% male) Age Monthly household income (1994 U.S. dollars) Educationb CRS~ Willingness to pay
U,S. 50 70.1 (7.95) 2671 (1975) 2.43 (0.60) 2.13 (1.89) $91 ($93 income-adjusted)d
Sweden [5] 46 59.6 (11.5) $2724a (1443) 1.33 (0.68) 5.08 (3.82) $117
~Updated to 1994 dollars using the Swedish consumer price index and current exchange rate. bCoded l for primary education, 2 for secondary education (completed high school), 3 for university education. CThe perceived value of hypertensive therapy, measured as the difference in perceived health with and without treatment on a category rating scale (CRS). Note that the CRS measured 15 cm for the Swedish study and l0 cm for the U.S. study. aThe adjusted U.S. WTP value uses the higher income of the Swedish group to facilitate comparison between the populations.
Willingness to pay for antihypertensive care that are acceptable and understandable to many respondents. Another option that may be preferred when feasible is administering WTP questions using trained interviewers. The lack of association between respondents' WTP and their prior history of stroke, myocardial infarction, and perceived present general health status was unexpected, although the coefficients for the individual events and the combined variable had the expected signs. Preferences for medical therapies are expected to show variation as a function of individual health status. Stroke and myocardial infarction increase the risk of future events and therefore increase the absolute risk reduction of hypertension treatment. It is likely that these coefficients did not achieve significance because the number of patients with these conditions was comparatively small. It is possible, however, that most of the myocardial infarction patients had little change in their overall health perception after the event. Some advocates of the WTP method have suggested that such surveys should be done by faceto-face interviews using a referendum format to minimize many of the problems that frequently arise when non-interviewer-administered techniques are used (U.S. Dept of Commerce, 1993; Mitchell and Carson, 1989). Because interviewer-administered surveys are time-consuming, resource-intensive, and subject to variabilities in interviewer training and experience, researchers have sought to develop alternative methods to obtain valid and reliable WTP estimates. Johannesson et al.'s binary bid WTP question reduces the likelihood of encountering starting point bias and range bias (Johannesson et al., 1993). In an earlier study of WTP for antihypertensive therapy by their group (Johannesson et al., 1991), an open-ended question format was used to avoid range and starting point bias; however, the open-ended question led to a large number of nonresponse and protest answers. Our bid format avoided open-ended responses, but had the disadvantage of being truncated at a maximum value. Since 9% of our respondents indicated that they would be willing to pay $250 per month for antihypertensive care, it is possible that a subset of this group would have paid more if higher values had been available as responses. Although starting point bias was not an issue in the more recent study Swedish study (Johannesson et al., 1993), it may have a problem with range bias, since the maximum possible response was set by the investigators. In that study. 24% responded "yes" to the maximum binary bid. In practice, the theoretical advantages of the binary bid method must be weighed against the programmatic difficulties of creating a multiple-format questionnaire, particularly when second or third mailings are used to improve response rates. In addition, the binary method employed by
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Johannesson et al. will generally require a larger sample size than is typically needed for health status surveys. Investigators may be unwilling or unable to sample more patients simply to accommodate the WTP question, unless WTP is the primary question that drives the study design. Another limitation of the study is that it surveys a relatively homogeneous population in a single insurance plan. There is evidence that preferences for specific health interventions may vary among different health insurance plans (Goddeeris, 1984; Grazier et al., 1986). If true, then it is possible that WTP for hypertension care might be different for individuals who are not insured by a staff-model HMO. From a policy perspective, this suggests that investigators in the United States may have to control for insurance status or benefit package in WTP evaluations for health care interventions, Multicollinearity between the age, education, and income variables may explain some of the problems with the explanatory power of the regression models. This issue was also noted in the Swedish study. A possible bias with WTP estimation is that the range of possible responses is capped at a level beyond which respondents are not able to indicate increasingly higher WTP values. Aggregation of responses at the highest value may lead to an underestimation of the true mean WTP. To adjust for this problem, Donaldson et al. (1995) have proposed a two-stage model to account for possible right censoring. We specified a two-stage model, using logit to estimate the instrument (first stage) and linear regression of the natural log values of WTP to estimate the second stage. The censoring bias coefficient estimated from the first stage was not statistically significant when used in the second stage, thus indicating either that our instrument was not correctly specified or that we did not have a censoring bias. In either case, the estimated mean WTP was generally the same. While this approach may be useful in cases where a large number of responses are at the ceiling value, in our study with proportionately few responses at the highest WTP category, it did not improve estimation. Willingness to pay has practical appeal because: (1) benefits are measured in monetary terms, thus permitting "stand alone" evaluation of a program, since net value is determined as the difference between benefits and costs; and (2) indirect and intangible benefits of treatment, which are typically difficult to quantify in cost-effectiveness studies, are incorporated into the monetary valuations of respondents. The WTP method is also theoretically appealing for use in cost-effectiveness analysis, because it avoids the problems that arise when costs and benefits are measured in different units. Despite these advantages, the WTP method suffers from a number of methodological and practical
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limitations (U.S. Dept of Commerce, 1993; Mitchell and Carson, 1989). First, to make an informed and accurate choice, respondents need near-perfect information regarding the health state, including its natural history and the costs and effectiveness of alternative therapies. In the case of this study, the important information that patients may lack is information about the risks of myocardial infarction and stroke with and without hypertension treatment. If this information is not clearly conveyed by physicians to patients, estimating the value of treating high blood pressure becomes extremely difficult, since the effects of the disease (stroke, myocardial infarction) are distant and catastrophic. In principle, it may be possible to give accurate treatmentassociated risk reduction information to patients, although estimating the present value of such information on a patient-specific basis is an admittedly difficult test. Second, even with adequate information, several factors may compel respondents to report inaccurately their WTP. For example, subjects who worry that their responses could affect their out-of-pocket medication capes might give falsely low W T P values. Finally, many argue that assigning a monetary value to human life and health is impossible or unethical. In response to these criticisms, Gafni has suggested that WTP questions should be framed in the context of hypothetical insurance purchasing, given that a service would be available if needed (Gafni, 1991). Despite these concerns, the findings from our study and the Swedish survey support the validity of W T P as a measure of benefits for hypertensive care. In addition to their similarity in adjusted mean response values, both studies are a relatively accurate estimate of the annual cost of hypertensive care. Published reports estimate the annual direct medical care cost of hypertension therapy in the United States to range from $411 to $1425 (1994 dollars), depending on the type of drug used for treatment (Hilleman et al., 1994; Odell and Gregory, 1995). In Sweden, the costs of hypertension treatment are lower ($400 500) (Johannesson et al., 1993). An unanswered question is how closely the costs of antihypertensive therapy match the present value of the expected future savings resulting from treatment-reduced rates of coronary disease and stroke (including indirect and intangible treatment benefits). We believe that these results support the notion that W T P is a valid and reliable method for measuring patient's perceived benefits from hypertensive care. Much of the criticism of W T P has grown out of studies of environmental legislation, which involve tasks of tremendous abstraction, such as the knowledge of the true environmental impact of the Exxon Valdez oil spill, for situations with which most individuals have no first-hand experience (U.S. Dept of Commerce, 1993). Patients know their own health. This study suggests that
they also know the consequences of their hypertension on their future health, and the value of the treatments they receive for this condition. REFERENCES
Donaldson, C., Shackley, P., Abdalla, M. and Miedzybrodzka, Z. (1995) Willingness to pay for antenatal carrier screening for cystic fibrosis. Journal of Health Economics 4(6), 439-452. Duan, N. (1983) Smearing estimate: a nonparametric retransformation method. Journal of the American Statistical Association 78, 605 610. Freyd, M. (1923) The graphic rating scale. Journal of Educational Psyehology 14, 83-102. Gafni, A. (1991) Willingness-to-pay as a measure of benefits: relevant questions in the context of public decisionmaking about health care programs. Medical Care 29, 246-1252. Goddeeris, J. H. (1984) Insurance and incentives for innovation in medical care. Southern Economic Journal 51, 530-539. Grazier, K., Richardson, D., Martin, D. and Diehr, P. (1986) Factors affecting choice of health care plans. Health Services Research 20, 659 682. Hilleman, D. E., Mohiuddin, S. M. and Lucas, B. D. (1994) Cost-minimization analysis of initial antihypertensive therapy in patients with mild-to-moderate essential diastolic hypertension. Clinical Therapeutics 16, 88-102. Johannesson, M. and Jonsson, B. (1991) Economics evaluation in health care: is there a role for cost-benefit analysis? Health Policy 20, 309. Johannesson, M., Jonsson, B, and Borgquist, L. (1991) Willingness to pay for antihypertensive therapy--results of a Swedish pilot study. Journal of Health Economics 10, 461 474. Johannesson, M., Johansson, P.-O. and Jonsson, B. (1992) Economic evaluation of drug therapy. Pharmacoeconomics 1,325 337. Johannesson, M., Johansson, P -O., Kristr6m, B. and Gerdtham, U. G. (1993) Willingness to pay for antihypertensive therapy--further results. Journal of Health Economies 12, 95 108. Mitchell, R. C. and Carson, R. T. (1989) Using Surveys to Value Public Goods." The Contingent Valuation Method.
Resources for the Future, Washington, DC. O'Brien, B. and Viramontes, J. L. (1994) Willingness to pay: a valid and reliable measure of health state preference? Medical Decision Making 14, 289-297. Odell, T. W. and Gregory, M. C. (1995) Cost of hypertension treatment. Journal of General Internal Medicine 10, 686-688. Scott, J. and Huskisson, E. C. (1977) Measurement of functional capacity with visual analogue scales. Rheumatology Rehabilitation 16, 257-259. U.S. Department of Commerce, National Oceanic and Atmospheric Administration (1993) Report of the NOAA panel on contingent valuation. Federal Register 58(10), 4601-4614.
APPENDIX A
This question concerns how much money you would be willing to pay for your high blood pressure care, including office visits and medicine. Assume for the moment that your insurance plan did not cover the costs of visits and medicine for high blood pressure. Assume that each of the dollar values listed below might represent your separate
Willingness to p a y for a n t i h y p e r t e n s i v e c a r e
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m o n t h l y bill f o r h i g h b l o o d p r e s s u r e care. F o r e a c h m o n t h l y cost, please circle the a n s w e r i n d i c a t i n g w h e t h e r o r n o t y o u w o u l d be willing to p a y t h a t a m o u n t o f m o n e y o u t o f y o u r o w n p o c k e t to c o n t i n u e y o u r t r e a t m e n t f o r h i g h b l o o d pressure. (Circle y o u r a n s w e r s . )
a. $25 a month b. $50 a month c. $75 a month d. $100 a month e, $125 a month f. $150 a month g. $175 a month h. $200 a month i. $225 a month j. $250 a month
Yes, definitely 1 1 1 1 1 1 1 1 I I
Yes, probably 2 2 2 2 2 2 2 2 2 2
No, probably not 3 3 3 3 3 3 3 3 3 3
No, definitely not 4 4 4 4 4 4 4 4 4 4
Don't know 5 5 5 5 5 5 5 5 5 5