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Using willingness to pay to value alternative models of antenatal care
Soc. Sci. Med. Vol. 44, No. 3, pp. 371-380, 1997
Pergamon S0277-9536(96)00154-2
Copyright © 1997 ElsevierScienceLtd Printed in Great Britain. All rights reserved 0277-9536/97 $17.00 + 0.00
USING WILLINGNESS TO PAY TO VALUE ALTERNATIVE MODELS OF ANTENATAL CARE M A N D Y R Y A N , ~ J U L I E R A T C L I F F E ~ and J A N E T T U C K E R 2 ~Health Economics Research Unit, University Medical Buildings, Foresterhill, Aberdeen, AB9 2ZD, Scotland, U.K. and 2Department of Epidemiolc,gy and Public Health, Ninewells Hospital and Medical School, Dundee, DD1 9SY, Scotland, U.K. Abstract--Recent years have seen the development of different models of antenatal care, especially for low risk women. More specifically, there has been a move for more general practitioner and midwifery involvement in such care. Given the current changes that are taking place in the provision of antenatal care, it is becoming increasingly important to carry out economic evaluations of alternative models of care. This paper applies the economic instrument of willingness to pay to assess the benefits of two alternative forms of antenatal care: general practitioner/midwife routine led care versus obstetrician led care. The results suggest a willingness to pay of £2500 for antenatal care, with no significant difference between the types of care provided. It is concluded that before firm policy conclusions can be reached, further studies should be undertaken to address methodological issues around the willingness to pay technique. Copyright © 1997 Elsevier Science Ltd Key words---closed-ended willing to pay, antenalal care
INTRODUCTION Given the current changes that are taking place in the provision of antenatal care, it is becomi~ag increasingly important to carry out economic evaluations of the developing alternative models. Recent changes in maternity services have led to models of antenatal care which aim to improve continuity of care and give women choice over such factors as location of care and who her main carer will be. The Scottish Office H o m e and Health Department Policy Review recommends a shift from consultant led care to midwife and general practitioner care. Whilst it is unlikely that clinical outcomes will vary across such models of care, women's preferences may. That is, women may have preferences for systems of care that provide more (or less) choice aJad continuity of care, as well as more (or less) involvement by midwives and general practitioners. An economic technique is therefore needed that is sensitive to measuring the value of such process attributes in the provision of care. In this paper the economic instrument of willivgness to pay (WTP) is applied to measure preferences for alternative models of antenatal care. In the next section a multicentre randomized controlled trial is described which aimed to evaluate two alternative forms of antenatal care: shared care versus general practitioner/midwife led care. This trial provided the sample for the application of the W T P technique. Information is also provided in this section on current economic evaluations of antenatal care. Following this, the rationale for using the economic instrument of W T P as a measure of the benefits of
antenatal care is discussed, and the W T P technique is described. Empirical work is then presented, showing the application of the W T P technique to compare the benefits from shared care versus general practitioner/ midwife led care. Results are presented and discussed. CURRENT PROVISION AND EVALUATIONOF ANTENATALCARE The introduction and widespread provision of antenatal care dates from the 1930s and has evolved to include techniques of fetal monitoring and prenatal diagnostic advances (Chamberlain, 1992). Shared antenatal care was shown to be the type of care delivered to 97% of Scottish women in late 1989 (Tucker et al., 1994). The lead provider in shared care is the obstetrician, but also working with the general practitioner and midwife. Throughout the 1980s there were consumer criticisms of the medicalization of child bearing and fragmentation of care (Foster, 1995). There were also questions by the medical professions about the effectiveness of care (Hall et al., 1980). These factors have encouraged the current development and evaluation of different models of maternity care provision--especially for women defined as at low risk of pregnancy complications. As a result, a multicentred randomized controlled trial of antenatal care for low risk women was conducted in Scotland in 1993-1994. It compared a new style of general practitioner/midwife care (hereafter called G P M care) in community settings with shared care. In the new style of care women receive all their routine appointments at their general practitioner surgery, from their general
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practitioner or community midwife. This is in contrast to shared care, where women receive additional routine appointments at the hospital where they are booked to deliver and thus their care is overseen by obstetricians in addition to general practitioners and midwives. The trial was administered by a multidisciplinary research team made up of clinicians, social scientists and health economists. The aim was to establish whether the new style of antenatal care was at least as safe as the old style of care, to compare women's satisfaction in the two arms of the trial and to compare the costs and benefits of the alternative styles of care. The methodology and results of the clinical evaluation, satisfaction component and costing study are reported elsewhere (Tucker et al., 1995; Ratcliffe et al., 1996). This paper concentrates on benefit assessment within the area of antenatal care. Most economic evaluations of antenatal care to date have concentrated on the benefits from prenatal screening alone. They have also assumed that the only benefit from such care arises as a result of improving the health of the child (Andreano and McCollum, 1983; Conley and Milunsky, 1975; Gill et al., 1987; Henderson, 1982; Hibbard et al., 1985; Moatti et al., 1989). All benefits are seen to flow from the detection and abortion of an abnormal foetus. Thus, any benefits women receive from the actual process of care have been ignored. In a previous paper in this journal Mooney and Lange (1993) criticized existing economic approaches to evaluating antenatal care screening for ignoring factors beyond health outcomes that are important to women in the provision of the service. The type of factors Mooney and Lange identified as potentially important were such attributes as the value of information and reassurance. When looking at the value of information, Mooney and Lange point out that such information may be of value to women who have an abnormal foetus detected and continue with the pregnancy as well as by women who detect and abort an abnormal foetus. With regard to reassurance, there may be value in knowing you have a normal foetus, as well as knowing you had done everything possible to prevent causing an abnormal foetus. They argue that a possible explanation for the exclusion of such factors in economic evaluations to date may relate to measurement difficulties. In looking at the benefits of the two alternative styles of care in the randomized trial, it was hypothesized in advance that whereas the clinical outcomes were unlikely to differ significantly between the two arms of the trial, the actual process of care would be different. [A priori hypotheses concerning clinical outcomes were confirmed in the trial (Tucker et al., 1995).] The new style of care would differ in process of care by: •
staff d e p l o y m e n t - - n o supervised visits;
routine
obstetrician
•
•
location--all routine visits would be at the G P clinic or health centres; and c h o i c e - - m o r e choice for women over time, place and supervisor of her care.
Attributes such as information and reassurance were also hypothesized to influence the value of antenatal care. However, the value of these attributes were not expected to be significantly different in the two arms of the trial. An economic technique was thus required that could assess the value of all such attributes as well as take account of different values in the two arms of the trial.
WILLINGNESS T O PAY
The rationale~or willingness to pay Cost-effectiveness was considered inappropriate to take account of such attributes, since this method uses a single outcome measure, which is usually clinical in nature, i.e. cost per healthy baby. The Quality Adjusted Life Year (QALY) approach was also considered inappropriate, since this approach has also concentrated on health outcomes (Williams, 1985; Boyle et al., 1983; Cadman et al., 1986; Feeny et al., 1994). Given that the main differences in this trial were likely to be in the actual process of care, rather than health outcomes, an economic technique sensitive to measuring preferences for alternative models of care that differ with regard to process attributes was required. The technique also needed to be sensitive enough to pick up the value of such attributes as information, reassurance and the value of knowing you have done everything possible to prevent an abnormal foetus. One technique that should allow for all such attributes in the valuation is the economic instrument of WTP. The technique of W T P is based on the premise that the maximum amount of money an individual is willing to pay for a commodity is an indicator of the utility or satisfaction to her of that commodity: political economy has to take as the measure of utility of an object the maximum sacrifice which each consumer would be willing to make in order to acquire the object... the only real utility is what people are willing to pay (Dupuit, 1844). The most obvious market where W T P behaviour is revealed is in auctions. Here individuals are pushed to consider the maximum amount of money they are willing to pay for a given commodity with given attributes. If the auction bid exceeds their maximum W T P they will drop out. When deciding on maximum WTP, they will take account of the characteristics or attributes of the commodity that are important to them. For example, in a housing auction the individual will consider such characteristics as number of rooms, location, whether centrally heated, whether double glazed and house type. Similarly in a car auction individuals will con-
Willingness to pay in antenatal care
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sider such characteristics as model, engine size, col- response rate (Cameron and Huppert, 1989; Arrow our and seating capacity when considering their et al., 1993). If the CV study is being carried out via an intermaximum WTP. Given that individuals are held to consider all the attributes that are important to view, the bidding technique can be used. This techthem in the provision of a good or service when nique is the oldest CV technique, used by Davis placing a monetary value on it, WTP was felt to be (1963) in the first application of WTP to hypothetiappropriate when considering the value of antenatal cal markets. Here individuals are asked if they care. In theory, when providing information on would be willing to pay a specified amount. If they maximum WTP for antenatal care, individuals answer "yes", interviewers increase the bid until should consider all the factors that are important to they reach amounts that the respondents are not them in the provision of the service. The valuation willing to pay. If they answer "no", the interviewer should therefore incorporate the value of clinical lowers the bid until they say "yes". WTP is estioutcomes, non-clinical outcomes (such as being bet- mated directly from the data provided. A variation on this, developed by Mitchell and ter informed, reassurance, and knowing you have done everything possible to prevent an abnormal Carson (1981), is the payment card CV technique. foetus) and process attributes (choice, location, con- Here respondents are presented with a range of bids tinuity of care and staff involved). A recent study and asked to circle the amount that represents the by Ryan (1995a) showed WTP to be sensitive to most they would be willing to pay. The individual's valuing such attributes. true maximum WTP will lie somewhere in the interMost markets are not characterized by auctions. val between the circled amount and the next highest Instead individuals are presented with a given price, option. While the questions here are held to be over which they have no influence, and faced with a easier to answer than in the open-ended approach, "take it or leave it" choice. In such a market the approach has the inherent problem that the revealed behaviour provides some information data provide only an interval estimation rather than about utility derived, though not necessarily m~txi- a point valuation. Such interval data are often anamum WTP. Occasionally, the market may be lysed by imputing maximum WTP as the mean characterized by a bidding type procedure between interval between the amount circled and the next the buyer and seller. Again, behaviour reveals some amount. information about utilities, though not necessa:dly The closed-ended CV (CECV) approach asks inmaximum WTP. dividuals whether they would pay a specified Some commodities are not marketed and do :not amount for a given commodity, with possible retherefore have an explicit money value in exchange. sponses being "yes" or "no". The bid amount is This is clearly the case for antenatal care in the varied across respondents and the only information National Health Service, where there is no cost obtained from each individual respondent is incurred at the point of consumption. Davis (1963) whether her maximum WTP is above or below the proposed the use of survey data to estimate WTP in bid offered (Bishop and Heberlein, 1979; Boyle and the absence of markets. Such surveys are commonly Bishop, 1984; Bishop and Boyle, 1985; Bowker and called contingent valuation (CV) surveys. Whilst Stoll, 1988). More recently this approach has been these have been widely used to assess the utility of extended by asking individuals two dichotomous environmental goods [for a review of the application questions, the second depending on the response to of CV in environmental economics see Mitchell and the first (Hanemann et al., 1991; Cameron and Carson (1989)], their use in health economics to Quiggin, 1994). Even three dichotomous choice date is limited [for a review of the application of questions have been asked, the second depending on CV in health economics see Donaldson (1993)]. the response to the first, and the third on the response to the second (Langford et al., 1994). It is Which willingness to pay question? also possible to allow for different degrees of cerContingent valuation studies have used a number tainty with respect to the responses (Johannesson et of techniques to assess maximum WTP, or utility, al., 1993). The CE approach has been recommended in the from hypothetical data. The four main ones are open-ended; bidding and payment card; and closed- environmental literature since it is held to more ended. Using the open-ended CV technique, respon- accurately reflect the decision that individuals make dents are asked directly what the maximum amount every day, i.e. individuals are presented with a cost, of money is that they would be prepared to pay for consider the attributes of the good or service, and a commodity. Advantages of this technique are that decide whether or not they want to purchase it WTP is easy to estimate and relatively small data (Arrow et al., 1993). Whilst the questions may more sets are needed. Maximum WTP is estimated using accurately reflect the decisions individuals make simple descriptive statistics such as the mean and every day, mean WTP is relatively more difficult to the median. The disadvantage of the technique is estimate and the approach requires a larger sample that respondents can find such open-ended ques- size than alternative WTP techniques to ensure tions difficult to answer, resulting in a high non- accurate estimation of WTP. The easiest way to SSM 44/3--D
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think about the estimation of WTP using the CE approach is to model the CE data as a demand function, where the bid level is modelled along the horizontal axis and the probability of saying yes along the vertical axis. From this it is possible to model the demand for a given good at a given price. Mean WTP can then be estimated as the area under this curve, and median WTP as the price at which 50% of the population would demand the good. Estimation of WTP using the CE approach can be modelled within the framework of random utility theory (RUT) (Hanemann, 1984). This involves first estimating the following function that will allow estimation of the probability that an individual will say "yes" to any given bid amount: R =f(Bid)
(1)
where the response, R, to a particular bid has a value of 1 if the respondent says yes and zero if she says no. The most common method used to estimate this equation is logit, which has the following form: P (Log) 1-Z~ = a + b.(Bid).
(2)
The predicted probability of saying "yes" can then be estimated as P = (1 + e (a+b.(aid)))-I
(3)
where a + b.Bid is the estimated regression logit equation (when only the bid amount is included as an independent variable) and P is the probability of accepting the bid. Mean WTP is estimated as the area under this probability function. This area shows the proportion of the population who would consume the good at each price level and their associated utility. The area under a curve is estimated by integration techniques and can be expressed as E(WTP) =
I
U(1 + e-a+b(bid ))- l db
O
-
l
O 1 - (1 + e-a+b(bid ))-I db L
(4)
where 1 + e-~" + h(bid))-' is the probability of saying "yes" and U and L the upper and lower limits of the integration, respectively. Estimating mean WTP within this framework relies on making some assumption about the upper and lower limits of the integral, i.e. knowing the bid amounts at which probability of saying "yes" is zero and probability of saying "yes" is one. Applying this to health care, and assuming that individuals will not use the health care intervention if they receive a disutility from it, negative WTP can be ruled out and zero used as the lower limit. Bishop and Heberlein (1979) and Sellar et al. (1985)
range of their bid amounts as the limit for the integration. Hanemann (1984) argued that such an approach makes assumptions about the probability distribution for the unknown (censored) WTP in the sample. He suggested that the upper limit should be infinity and that applying the highest offered amount as the upper limit may be a poor approximation to the mean utility estimated when integrating between zero and infinity. An alternative method is to truncate the integral at an offer corresponding to where a fixed percentage of respondents will refuse the bid. used the upper
upper
METHODS
Sample
The data were derived from a randomized trial comparing two alternative forms of antenatal care: GPM care versus shared care. A total of 1765 low risk women were recruited to this trial b e t w e e n February 1993 and March 1994 from 51 general practices linked to nine hospital centres throughout Scotland. To obtain information on WTP a questionnaire was mailed to all 936 women who had not delivered in the second week of November 1993. This week was chosen to obtain the maximum number of women recruited to the trial who had not yet delivered. (It was felt that the actual delivery experience would influence respondents' WTP and that valuations should be ex ante rather than ex post since this is when policy makers have to make decisions regarding the optimal allocation of scarce NHS resources.) Questionnaire
The questionnaire aimed to establish the value of the two alternative ways of providing antenatal care, taking account of process type attributes (e.g. involvement of different staff, location of antenatal care appointments and choice in the delivery of antenatal care) and non-clinical outcomes (e.g. provision of information and reassurance). The WTP technique was used since, when considering WTP, individuals are held to consider all the attributes of a service or service that are important to them. Given recommendations from the environmental literature (Arrow et al., 1993), the closed-ended (CE) WTP technique was used. (The questionnaire also collected information on costs incurred by women and their companions at their last antenatal care appointment.) One methodological issue involved in designing a CE WTP questionnaire is deciding what bid amounts to include, and how many individuals to allocate to each bid amount, i.e. deciding the bid vector. Cooper and Loomis (1992) have highlighted the importance of the bid vector in designing CE WTP studies. Initially it was decided to have £ amounts that varied from £25 to £900 (i.e. £25, £50,
Willingnessto pay in antenatal care
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Table 1. Final bid vector for randomized trial Level
Sent out
Returned yes
Returned no
P (YES)
£25 £50 £75 £100 £125 £150 £175 £200 £250 £300 £350 £400 £500 £600 £700 £900 £1500 a £3000 a £5000 a £10000 a
50 49 50 50 46 48 48 49 49 47 47 49 49 48 49 43 42 42 42 42
24 21 19 33 20 18 21 22 15 18 20 8 17 13 15 9 I1 11 5 3
12 12 10 12 14 15 14 14 22 12 16 23 26 21 20 14 16 19 23 20
0.67 0.64 0.65 0.73 0.59 0.54 0.60 0.61 0.40 0.60 0.55 0.26 0.51 0.38 0.42 0.39 0.40 0.36 0.17 0.13
aLevels added to the bid vector after initial bid vector showed that 39% of respondents were still saying "yes" at highest bid of £900.
£75, £100, £125, £150, £175, £200, £250, £300, £3.';0, £400, £500, £600, £700, £900). Equal numbers of individuals in each arm of the trial were randomly assigned to one of these amounts. As described previously, to estimate WTP it is important that a high percentage of individuals are saying no at the highest bid amount. Only then can the area under the curve be accurately estimated. However, as can be seen from Table 1, this was not the case; at the highest bid amount of £900, the probability of s~.ying yes was still 39%. Thus, more individuals Who had been recruited into the trial since the initial WTP questionnaire mailing were sent questionnaires with bid amounts of either £1500, £3000, £5000 or £10,000. Table 1 shows the final bid vector. For the wording of the WTP question see Appendix A. A statement was included in the WTP questionnaire telling respondents that there was no way that they would have to pay the bid amount they were offered and that the aim of the question was only to find out how much they value antenatal care. This statement was included in the wording of the WTP question for a number of reasons. Using the WTP technique in the area of health care in the British National Health Service is politically sensitive and has to be adapted accordingly. It is not like asking respondents' WTP for environmental goods. Experience of using the WTP technique in the British NHS has shown that providers of health care are concerned that including WTP questionnaires will lead to suggestions that they are attempting to privatize the health care system. Such providers have therefore insisted on such a statement being included if WTP is to be used as an instrument to value health care interventions. Inclusion of this statement was also intended to reduce "protest bids", i.e. respondents not answering the question because they have a moral obj,ection to being asked WTP for health care. It is recognized that such a statement may invite respon-
dents to overstate their WTP and that work is therefore needed on the validity of the WTP technique using this statement. However, in the only empirical study testing the "overpledging hypothesis", Bohm (1972) found no evidence of respondents overstating their WTP when they are told that they would not actually have to pay anything, no matter what they said they would be WTP. Bohm looked at respondents' WTP to see a preview of a Swedish television show. He was concerned with numerous aspects of strategic bias, including free-riding and overbidding. Respondents were allocated to one of five possible payment situations: actual payment of the WTP offered by each individual; payment amount proportional to WTP; lottery chooses payment method; fixed payment of 5 Kroner and no payment required. In the first two scenarios freeriding was hypothesized, in the third no strategic behaviour was expected, and in the latter two overbidding was expected. However, Bohm found that respondents were not sensitive to the strategic incentives included in the experiment, and that the WTP amounts revealed in the five scenarios were not statistically significantly at any accepted significant level.
Econometric analysis Logistic regression was used to estimate WTP. To test whether there was a significant difference in WTP across the two arms of the trial (GPM care versus shared care), arm of the trial was also included as an independent variable in the logit regression equation. This took on a value of zero if the woman was randomized to the new style of care and one if she was randomized to the old style of care. If there was a significant difference in the values individuals gave to each type of antenatal care, this would be expected to be reflected in the arm of the trial being a significant predictor of WTP (i.e. having a p value less than or equal to 0.05). Other variables which were hypothesized, a
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priori, to influence WTP for antenatal care were
also included in the logistic regression equation. These variables are shown in Table 2. Inclusion of such independent variables also allows for the testing of the internal validity of the WTP technique, i.e. the extent to which the results are consistent with economic theory, or, more generally, a priori expectations. For example, it was hypothesized, a priori, that the higher the level of satisfaction with previous experience of antenatal care in previous pregnancies, the greater would be WTP for current antenatal care. Similarly, the hypothesis behind inclusion of perceived risk of having problems with own health and babies health during pregnancy was that a greater perceived risk would lead to an increased WTP. It was hypothesized that the higher the income level, the higher would be WTP. There was no a priori hypotheses for inclusion of number of antenatal care appointments to date nor age of respondent. All variables were initially included in the model (the general model). A general to specific regression method was then used, with explanatory variables being excluded from the general model in a stepwise fashion. Using this method, the first variable excluded from the regression equation is the one with the smallest positive or negative correlation with the dependent variable. The F-test for the hypothesis that the coefficient of that variable is zero is then estimated. The variable is dropped from the equation only if the probability associated with the F-test is greater than or equal to 0.1. This variable is then examined and re-entered if the probability associated with the F-test is smaller than 0.05. Having carried out these tests on the first variable, all variables in the equation are again tested for exit and entry according to the above steps. This procedure continues until none of the variables remaining attains exit or entry criteria (Norusis, 1990). Given that a substantial proportion of respondents had no previous experience of antenatal care, and therefore did not provide information on satisfaction with antenatal care, the logistic regression equation was reran without satisfaction as an independent variable. A general and specific equation were again estimated.
In estimating mean WTP, explanatory variables are set equal to their sample means. A linear model was assumed since this is consistent with economic theory (Hanemann, 1984). The upper and lower limits of the integration were taken as zero to plus infinity since it is assumed that if individuals have a negative utility, they would choose not to have treatment. When estimating logistic models goodness of fit measures include the log-likelihoods, the chi-squared (which use the log-likelihood to test the overall significance of the model) and the proportion of correct predictions made by the estimated model compared with the original data. These statistics are all presented for the logistic models estimated in this paper.
RESULTS
Of the 956 questionnaires mailed, 704 were returned, giving a response rate of 74%. Of these respondents 658 (93%) completed the WTP section. This suggests that the WTP question is acceptable to respondents. Table 3 shows the results from both the general and specific models when satisfaction is included as an explanatory variable. In the specific model, all of the variables have the expected signs. The negative sign on the bid variable indicates as we would expect that the higher the bid amount offered to the respondent, the greater the probability that they will say no. The negative coefficient on age implies that older women place a lower value on antenatal care. The positive sign on satisfaction implies that the more satisfied women are with their antenatal care, the more they are willing to pay. This is what we would expect. The positive and significant sign on income provides support for the theoretical validity of the CE WTP technique, with WTP being positively related to income. Average WTP is estimated to be £2703 from the specific model, with no significant difference between the two arms of the trial. This suggests that arm of the trial does not have a significant impact on the valuation given to antenatal care. The number of observations in the initial model is only 312. This is due to the fact that a large num-
Table 2. Variables included in the logit regression model Description o f variable Bid level offered to individual A r m o f the trial (0 = G P M , 1 - shared care) Age o f respondent N u m b e r o f antenatal care appointments had so far during pregnancy Satisfaction with previous experience o f antenatal care (scale of 1 5 where 1 represents very dissatisfied and 5 very satisfied) H o w likely do you think you are to have problems with your health during your pregnancy (scale of 1-5 where I = very likely and 5 = very unlikely) H o w likely do you think you are to have problems with your baby duting your pregnancy (scale of 1-5 where I = very likely and 5 = very unlikely) Income (1 = less than £6000; 2 = £6000-£10,000; 3 = £10,001-£15,000; 4 = £15,001-£20,000; 5 = £20,001£25,000; 6 = £25,001-£30,000; 7 = £30,001-£35,000; 8 = greater than £35,000
Variable name BID ARM AGE NUMAPPT SATIS HEALTHPR PREGPROB INCOME
Willingness tc, p a y in a n t e n a t a l care
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Table 3. Logit regression results for general and specific model when including satisfaction General model Attribute
Coefficient
Constant BID ARM AGE NUMAPPT SATIS HEALTHPR PREGPROB INCOME n Log-likelihood McFadden R 2 ;(2 Correct predictions Mean WTP (£) 9 5 0 confidence intervals
2.0583 -0.0003 -0.0268 -0.0872 0.0276 0.5342 -0.1264 0.0470 0.4736 312 - 185 O. 14
Specific model P
Coefficient
P
0.0854 0.0005 0.9165 0.0249 0.4899 0.0024 0.4190 0.7918 0.0001 312 - 185
2.0260 -0.0003
0.0494 0.0005
-0.0913
0.0181
0.5245
0.0025
0.4657
0.0001
O. 14
62.16 (0.001) 68% 2548 1701-5183
60.96 (0.001) 68% 2703 1813-5772
ber of respondents had no experience of antenatal 93% of these respondents completing the WTP care previously and therefore could not provide question, suggested that individuals were willing to data on their level of satisfaction with previous ex- complete a CE WTP questionnaire. However, this perience of the service. In the next step the satisfac- high response rate may be partly explained by the tion variable was excluded from the logistic fact that respondents were taking part in a clinical regression analysis to increase the number of obser- trial. The positive and statistically significant coeffivations. Table 4 shows this model. This makes little cients on income and satisfaction were as expected difference to the results, with bid level, age ~,nd and provide support for the internal validity of the income level remaining statistically significant in the technique. same direction as above. Willingness to pay is now The findings suggest that average WTP for ante£2467, and there is again no significant difference in natal care is £2467 (for the model excluding the satWTP between the two arms of the trial. isfaction variable), with no significant difference between the arms of the trial. However, these findings should be interpreted with care for a number DISCUSSION AND CONCLUSION of reasons. Whilst using the CE approach may This paper used the CE WTP technique to esti- make the type of questions easier to answer than mate the value of alternative models of antenatal using the open-ended, payment card or bidding care. One of the main aims was to establish whether technique (and this is ultimately an empirical questhere was a significant difference in the valuation tion), there are still several methodological quesgiven by respondents in the two arms of the trial. tions around the analysis of CE data which need to By using the economic instrument of WTP, the be addressed. These include devising the bid vector study also attempted to take account of such altri- as well as considering the reliability and validity of butes as information, reassurance and knowing the technique. With regard to the bid vector, you have done everything to prevent an abnomaal Duffield and Paterson (1991) carried out sensitivity analysis on CE WTP estimates, using Monte Carlo foetus. The findings provide support for the use of the simulations. They kept total sample size and the bid CE WTP approach. The response rate of 74%, with values fixed, but reallocated the number of individTable 4. Logit regression results for general and specific model when excluding satisfaction General model Attribute Constant BID ARM AGE NUMAPPT HEALTHPR PREGPROB INCOME n Log-likelihood McFadden R z X: Correct predictions Mean WTP (£) 95% confidence intervals
Specific model
Coefficient
P
Coefficient
P
0.2180 -0.0004 0.1569 -0.0459 0.0203 -0.0715 0.0685 0.3139 563 -351 0.10 78.02 (0.0001) 67% 2354 1713-3854
0.7670 0.0001 0.3899 0.0681 0.4755 0.5470 0.6053 0.0001 563 -352 0.10 78.02 (0.0001) 67% 2467 1773-3884
0.4370 -0.0004
0.4593 0.0001
-0.0466
0.0613
0.3097
0.0001
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uals assigned to each bid level. Mean WTP was attributes. This approach has been found to be senfound to be sensitive to such reallocations. Cooper sitive to process and non-clinical outcomes in health and Loomis (1992) looked at the sensitivity of WTP care (Ryan and Farrar, 1994; Ryan, 1995a). The estimates to both the bid vector design alternative authors plan to apply conjoint analysis in future measures of WTP. They concluded that the analysis work looking at alternative models of antenatal of 10 different CE WTP data sets indicated a sensi- care. tivity in estimated WTP to the bid vector design From a policy perspective, the results from the and the method chosen to estimate WTP. costing part of the trial show that GPM care costs Regarding reliability and validity, whilst evidence society statistically significantly less than shared from the environmental literature supports the re- care (Ratcliffe et al., 1996). Clinical outcomes and liability (Jones-Lee et al., 1985; Heberlein, 1986; consumer satisfaction were shown to be the same in Loomis, 1988; Loehman and De, 1992) and validity the two arms of the trial (Tucker et al., 1995), and (Cummings et al., 1986; O'Brien and Viramontes, the results of the WTP study presented here indicate 1994) of the WTP approach, it is not clear at the that there is no difference in strength of consumer moment whether these results are transferable to preference between the two alternative forms of health care. The commodity of health care clearly care. As such, GPM antenatal care would be the differs in many ways from many of the commodities efficient option (compared to shared care) for routo which the technique of CV has been applied in tine antenatal care for low risk pregnant women the environmental literature. Studies are thus throughout Scotland. needed to assess the reliability and validity of the technique within health care. Acknowledgements--This study was funded by the Health Secondly, the insignificance of the arm of the trial Services and Public Health Research Committee of the in the regression equation may reflect the insensitiv- Chief Scientist Office of the Scottish Office Home and ity of the WTP instrument to pick up the import- Health Department (grant number K\OPR\2\2D63), but ance of particular process attributes. In the the views expressed are those of the authors. We thank all the women who completed the questionnaire and Cam environmental literature there has been a big debate Donaldson for comments made on earlier drafts of the about whether CV is sensitive to the size of the pro- paper. gramme being evaluated. For example, Boyle et al. (1994) found that people's WTP to take action to REFERENCES prevent the death of 2000 migratory birds in an oil spill is the same as their WTP to prevent 20,000 and Andreano, R. L. and McCollum, D. W. (1983) A benefit 200,000 such deaths. This is known in the environcost analysis of amniocentesis. Social Biology 30, 347metal literature as the embedding effect and results 373. when the value attached to a general good is the Arrow, K., Solow, R., Portney, P., Learner, E., Radner, R. and Schuman, H. (1993) Report of the NOAA same as that attached to a specific form of the panel of contingent valuation. Federal Register 10, good. Within the antenatal study reported here this 4601-4614. embedding effect may manifest itself by the respon- Bishop, R. C. and Boyle, K. J. (1985) The Economic Value of Illinois Beach State Nature Reserve. Report to the dent expressing a WTP value for antenatal care Illinois Department of Conservation Madison, Heberlein overall rather than for the specific type of care. If and Baumgartner Research Services, Illinois. the most important factor to users is actually under- Bishop, R. and Heberlein, T. A. (1979) Measuring values taking care, then it is not surprising that monetary of extra market goods: are indirect measures biased? American Journal of Agricultural Economics 61,926-930. values do not significantly differ according to arm of trial. This has been suggested elsewhere within Bohm, P. (1972) Estimating demand for public goods: an experiment. European Economic Review 3, 111-130. the context of randomized controlled trials Bowker, J. and Stoll, J. (1988) Use of dichotomous choice (Donaldson et al., 1995). The authors of that paper nonmarket methods to value the whooping crane argue that to establish the utility attached to differresource. American Journal of Agricultural Economics 70, 373-381. ent aspects of the process of treatment, respondents could be given descriptions of the two alternative Boyle, K. and Bishop, R. (1984) Economic" Benefits Associated with Boating and Canoeing on the Lower types of care, asked which one they prefer, and then Wisconsin River. Economic Issues No. 84, Department asked to state the extra amount they are willing to of Agricultural Economics, University of Wisconsin, pay for their preferred option over their less preMadison. ferred option. An alternative approach would be to Boyle, M., Torrance, G., Sinclair, J. and Horwood, S. (1983) Economic evaluation of neonatal intensive care use the economic instrument of conjoint analysis of very low birth weight infants. New England Journal of (Ryan, 1995b). Using this latter technique, individMedicine 308, 1330-1337. uals are presented with alternative ways of provid- Boyle, K. J., Desvouges, W. H., Johnson, F. R., Dunford, R. W. and Hudson, S. P. (1994) An investigation of ing a good or service which vary with respect to part whole biases in contingent valuation studies. attributes of interest and are asked to state their Journal of Environmental Economics and Management preferences for such goods or services through rank27, 64-83. ing, rating or pairwise choice exercises. Thus, indi- Cadman, D., Goldsmith, C., Torrance, G., Boyle, M. and viduals explicitly consider the value of different Furlong, W. (1986) Development of a Health Status
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Index for Ontario Children. Ontario Ministry of Health, Johannesson, M., Johansson, P.-O., Kristrom, B. and Gerdtham, U.-G. (1993) Willingness to pay for antihyOntario. pertensive therapy--further results. Journal of Health Cameron, T. A. and Huppert, D. D. (1989) OLS versus Economics 12, 95-108. ML estimation of non-market resource values with payment card interval data. Journal of Enviromental Jones-Lee, M., Hammerton, M. and Philips, P. R. (1985) The value of safety: results of a national sample survey. Economics and Management 17, 230-246. The Economic Journal 95, 49-72. Cameron, T. and Quiggin, J. (1994) Estimation using contingent valuation data from a "dichotomous choice with Langford, I., Bateman, I. and Langford, H. (1994) Multilevel Modelling and Contingent Valuation. Part 1: A follow-up" questionnaire. Journal of Environmental Triple Bounded Dichotomous Choice Analysis. Working Economics and Management 27, 218-234. Paper No. GEC 94-04, Centre for Social and Economic Chamberlain, G. (1992) ABC of Antenatal Care. BMJ Research on the Global Environment, University of Publication, London. East Anglia and University College London. Conley, R. and Milunsky, A. (1975) The economics of prenatal genetic diagnosis. In The Prevention of Genetic Loehman, E. T. and De, V. H. (1992) Application of stochastic choice modeling to policy analysis of public Disease and Mental Retardation, ed. A. Milunsky, pp. goods: a case study of air quality improvements. Review 442-455. Saunders, Philadelphia. of Economics and Statistics 64, 474-480. Cooper, J. and Loomis, J. (1992) Sensitivity of willingness to pay to bid design in dichotomous choice contingent Loomis, J. (1988) Contingent valuation using dichotomous choice models. Journal of Leisure Research 20, 45-56. valuation models. Land Economics 68, 211 222. Cummings, R. G., Brookshire, D. S. and Schulze, W. D. Mitchell, R. C. and Carson, R. T. (1981) An Experiment in Determining Willingness to Pay for National Water (1986) Valuing Environmental Goods: A State of the Arts Quality Improvements. Report to the U.S. Assessment of the Contingent Method. Rowman and Environmental Protection Agency, Washington. Allanheld, Totowa. Davis, R. K. (1963) Recreation planning as an economic Mitchell, R. C. and Carson, R. T. (1989) Using Surveys to Value Public Goods. Resources for the Future, problem. Natural Resources Journal 3, 239-249. Washington. Donaldson, C. (1993) Theory and Practice of Willingness to Pay for Health Care. Health Economics Resea::ch Moatti, J. P., Lanoe, J. L., Le Gale, G. C., Gardent, H., Julian, C. and Ayme, S. (1989) Economic assessment of Unit Discussion Paper No. 01/93, University of prenatal screening in France: regulation of diffusion Aberdeen, Aberdeen. through inequities of access. Paper presented to the Donaldson, C., Shackley, P., Abdalla, M. z.nd Joint Meeting of the European Health Economists Miedzybrodzka, Z. (1995) Willingness to pay for acuteSocieties, Barcelona. natal carrier screening for cystic fibrosis. Health Mooney, G. and Lange, M. (1993) Antenatal screening: Economics 4, 439-452. what constitutes "benefit"? Social Science & Medicine Duffield, J. and Paterson, D. (1991) Inference and optimal 37, 873-878. design for a welfare measure in dichotomous choice conNorusis, M. J. (1990) SPSS Statistical Data Analysis. tingent valuation. Land Economics 67, 225-239. SPSS Inc, Chicago. Dupuit, J. (1844) De la mesure de l'utilite des travaux p~abO'Brien, B. and Viramontes, J. (1994) A valid and reliable lics. Annales des Ponts et Chaussees 8, 79-84 [Translated measure of health state preference? Medical Decision by Barback, R. H. (1952) On the measurement of utility Maker 14, 289-297. of public works. International Economic Papers 2, 1t3Ratcliffe, J., Ryan, M. and Tucker, J. (1996) The "costs" 110]. of alternative types of antenatal care for low risk Feeny, D., Torrance, G., Goldsmith, C., Furlong, W. ~md women--shared care versus community care by general Boyle, M. (1994) A Multi-attribute Approach to practitioners and midwives. Journal of Health ,Services Population Health Status. Working Paper No. 94-5, Research and Policy 1, 135-140. McMaster University, Ontario. Ryan, M. (1995a) Using Conjoint Analysis to Go Beyond Foster, P. (1995) Women and the Health Care Industry: An Health Outcomes: An Application to In Vitro Unhealthy Relationship. Open University Press, Fertilisation. HERU Discussion Paper No. 09/95, Buckingham. University of Aberdeen, Aberdeen. Gill, M., Murday, V. and Slack, J. (1987) An economic Ryan, M. (1995b) Economics and the patient's utility funcappraisal of screening for Down's syndrome in plegtion: an application to assisted reproductive techniques. nancy using maternal age and serum alpha fetoprolein Ph.D. thesis, University of Aberdeen. concentration. Social Science & Medicine 24, 725-731. Ryan, M. and Farrar, S. (1994) A Pilot Stud), Using Hall, K. K., Chng, P. K. and MacGillivray, I. (1980~, Is Conjoint Analysis to Establish the Views of Users in the routine antenatal care worthwhile? Lancet 1, 78-80. Provision of Orthodontic Services in Grampian. Health Hanemann, W. (1984) Welfare evaluations in contingent Economics Research Unit Discussion Paper No. 07/94, valuation experiments with discrete responses: reply. University of Aberdeen, Aberdeen. American Journal of Agricultural Economics 69, 332-341. Sellar, C., Chavas, J. P. and Stoll, J. R. (1985) Validation Hanemann, M., Loomis, J. and Kanninen, B. (1991) of empirical measures of welfare change: a comparison Statistical efficiency of double-bounded dichotomous of nonmarket techniques. Land Economics 61, 156-175. choice contingent valuation. American Journal of Tucker, J., Florey, C., Howie, P. W., Hall, M. H. and Agricultural Economics 73, 1255-1263. Mcllwaine, G. M. (1994) Is antenatal care apportioned Heberlein, T. A. (1986) Measuring resource values: the reaccording to obstetric risk? The Scottish antenatal care liability and validity of dichotomous contingent valuastudy. Journal of Public Health Medicine 16, 60-70. tion measures. Paper presented at the American Tucker, J., Howie, P. W., Hall, M. H., Mcllwaine, G. M. Sociological Association Meeting, New York. and Florey, C. du V. (1995) Should Obstetricians See Henderson, J. B. (1982) An economic appraisal of the benWomen with Normal Pregnancies? A Multi Centre efits of screening for open spina bifida. Social Science & Randomised Controlled Trial of Routine Antenatal Care Medicine 16, 545-560. by GPs and Midwives compared to Obstetrician Led Hibbard, B. M., Roberts, C. J., Elder, G. H., Evans, K. T. Care. Report to Scottish Office Health Department, Edinburgh. Laurence, K. M. (1985) Can we afford screening for neural tube defects? The South Wales experience. British Williams, A. (1985) Economics of coronary artery bypass grafting. British Medical Journal 291, 326-329. Medical Journal 290, 293-295.
Mandy Ryan et al.
380 APPENDIX A
W T P Question
In this section I am interested in the value you place on antenatal care. One way of finding out how much you value antenatal care is to find out the maximum amount of money you would be willing to pay for the provision of the service if it were not freely available on the NHS. THERE IS NO WAY YOU WOULD HAVE TO PAY FOR THIS SERVICE. THE AIM OF THE QUESTION IS ONLY TO FIND OUT HOW MUCH YOU VALUE ANTENATAL CARE.
22. Would you be willing to pay £10,000 for antenatal care during your pregnancy? Imagine the amount could be paid by extra taxation or through a voluntary donation. Also remember that any contribution made would reduce what you have left to spend on yourself. So would you be willing to pay £10,000' for antenatal care during your pregnancy? Yes
~
No *Bid amount varied from £25 to £10,000
Pergamon S0277-9536(96)00154-2
Copyright © 1997 ElsevierScienceLtd Printed in Great Britain. All rights reserved 0277-9536/97 $17.00 + 0.00
USING WILLINGNESS TO PAY TO VALUE ALTERNATIVE MODELS OF ANTENATAL CARE M A N D Y R Y A N , ~ J U L I E R A T C L I F F E ~ and J A N E T T U C K E R 2 ~Health Economics Research Unit, University Medical Buildings, Foresterhill, Aberdeen, AB9 2ZD, Scotland, U.K. and 2Department of Epidemiolc,gy and Public Health, Ninewells Hospital and Medical School, Dundee, DD1 9SY, Scotland, U.K. Abstract--Recent years have seen the development of different models of antenatal care, especially for low risk women. More specifically, there has been a move for more general practitioner and midwifery involvement in such care. Given the current changes that are taking place in the provision of antenatal care, it is becoming increasingly important to carry out economic evaluations of alternative models of care. This paper applies the economic instrument of willingness to pay to assess the benefits of two alternative forms of antenatal care: general practitioner/midwife routine led care versus obstetrician led care. The results suggest a willingness to pay of £2500 for antenatal care, with no significant difference between the types of care provided. It is concluded that before firm policy conclusions can be reached, further studies should be undertaken to address methodological issues around the willingness to pay technique. Copyright © 1997 Elsevier Science Ltd Key words---closed-ended willing to pay, antenalal care
INTRODUCTION Given the current changes that are taking place in the provision of antenatal care, it is becomi~ag increasingly important to carry out economic evaluations of the developing alternative models. Recent changes in maternity services have led to models of antenatal care which aim to improve continuity of care and give women choice over such factors as location of care and who her main carer will be. The Scottish Office H o m e and Health Department Policy Review recommends a shift from consultant led care to midwife and general practitioner care. Whilst it is unlikely that clinical outcomes will vary across such models of care, women's preferences may. That is, women may have preferences for systems of care that provide more (or less) choice aJad continuity of care, as well as more (or less) involvement by midwives and general practitioners. An economic technique is therefore needed that is sensitive to measuring the value of such process attributes in the provision of care. In this paper the economic instrument of willivgness to pay (WTP) is applied to measure preferences for alternative models of antenatal care. In the next section a multicentre randomized controlled trial is described which aimed to evaluate two alternative forms of antenatal care: shared care versus general practitioner/midwife led care. This trial provided the sample for the application of the W T P technique. Information is also provided in this section on current economic evaluations of antenatal care. Following this, the rationale for using the economic instrument of W T P as a measure of the benefits of
antenatal care is discussed, and the W T P technique is described. Empirical work is then presented, showing the application of the W T P technique to compare the benefits from shared care versus general practitioner/ midwife led care. Results are presented and discussed. CURRENT PROVISION AND EVALUATIONOF ANTENATALCARE The introduction and widespread provision of antenatal care dates from the 1930s and has evolved to include techniques of fetal monitoring and prenatal diagnostic advances (Chamberlain, 1992). Shared antenatal care was shown to be the type of care delivered to 97% of Scottish women in late 1989 (Tucker et al., 1994). The lead provider in shared care is the obstetrician, but also working with the general practitioner and midwife. Throughout the 1980s there were consumer criticisms of the medicalization of child bearing and fragmentation of care (Foster, 1995). There were also questions by the medical professions about the effectiveness of care (Hall et al., 1980). These factors have encouraged the current development and evaluation of different models of maternity care provision--especially for women defined as at low risk of pregnancy complications. As a result, a multicentred randomized controlled trial of antenatal care for low risk women was conducted in Scotland in 1993-1994. It compared a new style of general practitioner/midwife care (hereafter called G P M care) in community settings with shared care. In the new style of care women receive all their routine appointments at their general practitioner surgery, from their general
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practitioner or community midwife. This is in contrast to shared care, where women receive additional routine appointments at the hospital where they are booked to deliver and thus their care is overseen by obstetricians in addition to general practitioners and midwives. The trial was administered by a multidisciplinary research team made up of clinicians, social scientists and health economists. The aim was to establish whether the new style of antenatal care was at least as safe as the old style of care, to compare women's satisfaction in the two arms of the trial and to compare the costs and benefits of the alternative styles of care. The methodology and results of the clinical evaluation, satisfaction component and costing study are reported elsewhere (Tucker et al., 1995; Ratcliffe et al., 1996). This paper concentrates on benefit assessment within the area of antenatal care. Most economic evaluations of antenatal care to date have concentrated on the benefits from prenatal screening alone. They have also assumed that the only benefit from such care arises as a result of improving the health of the child (Andreano and McCollum, 1983; Conley and Milunsky, 1975; Gill et al., 1987; Henderson, 1982; Hibbard et al., 1985; Moatti et al., 1989). All benefits are seen to flow from the detection and abortion of an abnormal foetus. Thus, any benefits women receive from the actual process of care have been ignored. In a previous paper in this journal Mooney and Lange (1993) criticized existing economic approaches to evaluating antenatal care screening for ignoring factors beyond health outcomes that are important to women in the provision of the service. The type of factors Mooney and Lange identified as potentially important were such attributes as the value of information and reassurance. When looking at the value of information, Mooney and Lange point out that such information may be of value to women who have an abnormal foetus detected and continue with the pregnancy as well as by women who detect and abort an abnormal foetus. With regard to reassurance, there may be value in knowing you have a normal foetus, as well as knowing you had done everything possible to prevent causing an abnormal foetus. They argue that a possible explanation for the exclusion of such factors in economic evaluations to date may relate to measurement difficulties. In looking at the benefits of the two alternative styles of care in the randomized trial, it was hypothesized in advance that whereas the clinical outcomes were unlikely to differ significantly between the two arms of the trial, the actual process of care would be different. [A priori hypotheses concerning clinical outcomes were confirmed in the trial (Tucker et al., 1995).] The new style of care would differ in process of care by: •
staff d e p l o y m e n t - - n o supervised visits;
routine
obstetrician
•
•
location--all routine visits would be at the G P clinic or health centres; and c h o i c e - - m o r e choice for women over time, place and supervisor of her care.
Attributes such as information and reassurance were also hypothesized to influence the value of antenatal care. However, the value of these attributes were not expected to be significantly different in the two arms of the trial. An economic technique was thus required that could assess the value of all such attributes as well as take account of different values in the two arms of the trial.
WILLINGNESS T O PAY
The rationale~or willingness to pay Cost-effectiveness was considered inappropriate to take account of such attributes, since this method uses a single outcome measure, which is usually clinical in nature, i.e. cost per healthy baby. The Quality Adjusted Life Year (QALY) approach was also considered inappropriate, since this approach has also concentrated on health outcomes (Williams, 1985; Boyle et al., 1983; Cadman et al., 1986; Feeny et al., 1994). Given that the main differences in this trial were likely to be in the actual process of care, rather than health outcomes, an economic technique sensitive to measuring preferences for alternative models of care that differ with regard to process attributes was required. The technique also needed to be sensitive enough to pick up the value of such attributes as information, reassurance and the value of knowing you have done everything possible to prevent an abnormal foetus. One technique that should allow for all such attributes in the valuation is the economic instrument of WTP. The technique of W T P is based on the premise that the maximum amount of money an individual is willing to pay for a commodity is an indicator of the utility or satisfaction to her of that commodity: political economy has to take as the measure of utility of an object the maximum sacrifice which each consumer would be willing to make in order to acquire the object... the only real utility is what people are willing to pay (Dupuit, 1844). The most obvious market where W T P behaviour is revealed is in auctions. Here individuals are pushed to consider the maximum amount of money they are willing to pay for a given commodity with given attributes. If the auction bid exceeds their maximum W T P they will drop out. When deciding on maximum WTP, they will take account of the characteristics or attributes of the commodity that are important to them. For example, in a housing auction the individual will consider such characteristics as number of rooms, location, whether centrally heated, whether double glazed and house type. Similarly in a car auction individuals will con-
Willingness to pay in antenatal care
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sider such characteristics as model, engine size, col- response rate (Cameron and Huppert, 1989; Arrow our and seating capacity when considering their et al., 1993). If the CV study is being carried out via an intermaximum WTP. Given that individuals are held to consider all the attributes that are important to view, the bidding technique can be used. This techthem in the provision of a good or service when nique is the oldest CV technique, used by Davis placing a monetary value on it, WTP was felt to be (1963) in the first application of WTP to hypothetiappropriate when considering the value of antenatal cal markets. Here individuals are asked if they care. In theory, when providing information on would be willing to pay a specified amount. If they maximum WTP for antenatal care, individuals answer "yes", interviewers increase the bid until should consider all the factors that are important to they reach amounts that the respondents are not them in the provision of the service. The valuation willing to pay. If they answer "no", the interviewer should therefore incorporate the value of clinical lowers the bid until they say "yes". WTP is estioutcomes, non-clinical outcomes (such as being bet- mated directly from the data provided. A variation on this, developed by Mitchell and ter informed, reassurance, and knowing you have done everything possible to prevent an abnormal Carson (1981), is the payment card CV technique. foetus) and process attributes (choice, location, con- Here respondents are presented with a range of bids tinuity of care and staff involved). A recent study and asked to circle the amount that represents the by Ryan (1995a) showed WTP to be sensitive to most they would be willing to pay. The individual's valuing such attributes. true maximum WTP will lie somewhere in the interMost markets are not characterized by auctions. val between the circled amount and the next highest Instead individuals are presented with a given price, option. While the questions here are held to be over which they have no influence, and faced with a easier to answer than in the open-ended approach, "take it or leave it" choice. In such a market the approach has the inherent problem that the revealed behaviour provides some information data provide only an interval estimation rather than about utility derived, though not necessarily m~txi- a point valuation. Such interval data are often anamum WTP. Occasionally, the market may be lysed by imputing maximum WTP as the mean characterized by a bidding type procedure between interval between the amount circled and the next the buyer and seller. Again, behaviour reveals some amount. information about utilities, though not necessa:dly The closed-ended CV (CECV) approach asks inmaximum WTP. dividuals whether they would pay a specified Some commodities are not marketed and do :not amount for a given commodity, with possible retherefore have an explicit money value in exchange. sponses being "yes" or "no". The bid amount is This is clearly the case for antenatal care in the varied across respondents and the only information National Health Service, where there is no cost obtained from each individual respondent is incurred at the point of consumption. Davis (1963) whether her maximum WTP is above or below the proposed the use of survey data to estimate WTP in bid offered (Bishop and Heberlein, 1979; Boyle and the absence of markets. Such surveys are commonly Bishop, 1984; Bishop and Boyle, 1985; Bowker and called contingent valuation (CV) surveys. Whilst Stoll, 1988). More recently this approach has been these have been widely used to assess the utility of extended by asking individuals two dichotomous environmental goods [for a review of the application questions, the second depending on the response to of CV in environmental economics see Mitchell and the first (Hanemann et al., 1991; Cameron and Carson (1989)], their use in health economics to Quiggin, 1994). Even three dichotomous choice date is limited [for a review of the application of questions have been asked, the second depending on CV in health economics see Donaldson (1993)]. the response to the first, and the third on the response to the second (Langford et al., 1994). It is Which willingness to pay question? also possible to allow for different degrees of cerContingent valuation studies have used a number tainty with respect to the responses (Johannesson et of techniques to assess maximum WTP, or utility, al., 1993). The CE approach has been recommended in the from hypothetical data. The four main ones are open-ended; bidding and payment card; and closed- environmental literature since it is held to more ended. Using the open-ended CV technique, respon- accurately reflect the decision that individuals make dents are asked directly what the maximum amount every day, i.e. individuals are presented with a cost, of money is that they would be prepared to pay for consider the attributes of the good or service, and a commodity. Advantages of this technique are that decide whether or not they want to purchase it WTP is easy to estimate and relatively small data (Arrow et al., 1993). Whilst the questions may more sets are needed. Maximum WTP is estimated using accurately reflect the decisions individuals make simple descriptive statistics such as the mean and every day, mean WTP is relatively more difficult to the median. The disadvantage of the technique is estimate and the approach requires a larger sample that respondents can find such open-ended ques- size than alternative WTP techniques to ensure tions difficult to answer, resulting in a high non- accurate estimation of WTP. The easiest way to SSM 44/3--D
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think about the estimation of WTP using the CE approach is to model the CE data as a demand function, where the bid level is modelled along the horizontal axis and the probability of saying yes along the vertical axis. From this it is possible to model the demand for a given good at a given price. Mean WTP can then be estimated as the area under this curve, and median WTP as the price at which 50% of the population would demand the good. Estimation of WTP using the CE approach can be modelled within the framework of random utility theory (RUT) (Hanemann, 1984). This involves first estimating the following function that will allow estimation of the probability that an individual will say "yes" to any given bid amount: R =f(Bid)
(1)
where the response, R, to a particular bid has a value of 1 if the respondent says yes and zero if she says no. The most common method used to estimate this equation is logit, which has the following form: P (Log) 1-Z~ = a + b.(Bid).
(2)
The predicted probability of saying "yes" can then be estimated as P = (1 + e (a+b.(aid)))-I
(3)
where a + b.Bid is the estimated regression logit equation (when only the bid amount is included as an independent variable) and P is the probability of accepting the bid. Mean WTP is estimated as the area under this probability function. This area shows the proportion of the population who would consume the good at each price level and their associated utility. The area under a curve is estimated by integration techniques and can be expressed as E(WTP) =
I
U(1 + e-a+b(bid ))- l db
O
-
l
O 1 - (1 + e-a+b(bid ))-I db L
(4)
where 1 + e-~" + h(bid))-' is the probability of saying "yes" and U and L the upper and lower limits of the integration, respectively. Estimating mean WTP within this framework relies on making some assumption about the upper and lower limits of the integral, i.e. knowing the bid amounts at which probability of saying "yes" is zero and probability of saying "yes" is one. Applying this to health care, and assuming that individuals will not use the health care intervention if they receive a disutility from it, negative WTP can be ruled out and zero used as the lower limit. Bishop and Heberlein (1979) and Sellar et al. (1985)
range of their bid amounts as the limit for the integration. Hanemann (1984) argued that such an approach makes assumptions about the probability distribution for the unknown (censored) WTP in the sample. He suggested that the upper limit should be infinity and that applying the highest offered amount as the upper limit may be a poor approximation to the mean utility estimated when integrating between zero and infinity. An alternative method is to truncate the integral at an offer corresponding to where a fixed percentage of respondents will refuse the bid. used the upper
upper
METHODS
Sample
The data were derived from a randomized trial comparing two alternative forms of antenatal care: GPM care versus shared care. A total of 1765 low risk women were recruited to this trial b e t w e e n February 1993 and March 1994 from 51 general practices linked to nine hospital centres throughout Scotland. To obtain information on WTP a questionnaire was mailed to all 936 women who had not delivered in the second week of November 1993. This week was chosen to obtain the maximum number of women recruited to the trial who had not yet delivered. (It was felt that the actual delivery experience would influence respondents' WTP and that valuations should be ex ante rather than ex post since this is when policy makers have to make decisions regarding the optimal allocation of scarce NHS resources.) Questionnaire
The questionnaire aimed to establish the value of the two alternative ways of providing antenatal care, taking account of process type attributes (e.g. involvement of different staff, location of antenatal care appointments and choice in the delivery of antenatal care) and non-clinical outcomes (e.g. provision of information and reassurance). The WTP technique was used since, when considering WTP, individuals are held to consider all the attributes of a service or service that are important to them. Given recommendations from the environmental literature (Arrow et al., 1993), the closed-ended (CE) WTP technique was used. (The questionnaire also collected information on costs incurred by women and their companions at their last antenatal care appointment.) One methodological issue involved in designing a CE WTP questionnaire is deciding what bid amounts to include, and how many individuals to allocate to each bid amount, i.e. deciding the bid vector. Cooper and Loomis (1992) have highlighted the importance of the bid vector in designing CE WTP studies. Initially it was decided to have £ amounts that varied from £25 to £900 (i.e. £25, £50,
Willingnessto pay in antenatal care
375
Table 1. Final bid vector for randomized trial Level
Sent out
Returned yes
Returned no
P (YES)
£25 £50 £75 £100 £125 £150 £175 £200 £250 £300 £350 £400 £500 £600 £700 £900 £1500 a £3000 a £5000 a £10000 a
50 49 50 50 46 48 48 49 49 47 47 49 49 48 49 43 42 42 42 42
24 21 19 33 20 18 21 22 15 18 20 8 17 13 15 9 I1 11 5 3
12 12 10 12 14 15 14 14 22 12 16 23 26 21 20 14 16 19 23 20
0.67 0.64 0.65 0.73 0.59 0.54 0.60 0.61 0.40 0.60 0.55 0.26 0.51 0.38 0.42 0.39 0.40 0.36 0.17 0.13
aLevels added to the bid vector after initial bid vector showed that 39% of respondents were still saying "yes" at highest bid of £900.
£75, £100, £125, £150, £175, £200, £250, £300, £3.';0, £400, £500, £600, £700, £900). Equal numbers of individuals in each arm of the trial were randomly assigned to one of these amounts. As described previously, to estimate WTP it is important that a high percentage of individuals are saying no at the highest bid amount. Only then can the area under the curve be accurately estimated. However, as can be seen from Table 1, this was not the case; at the highest bid amount of £900, the probability of s~.ying yes was still 39%. Thus, more individuals Who had been recruited into the trial since the initial WTP questionnaire mailing were sent questionnaires with bid amounts of either £1500, £3000, £5000 or £10,000. Table 1 shows the final bid vector. For the wording of the WTP question see Appendix A. A statement was included in the WTP questionnaire telling respondents that there was no way that they would have to pay the bid amount they were offered and that the aim of the question was only to find out how much they value antenatal care. This statement was included in the wording of the WTP question for a number of reasons. Using the WTP technique in the area of health care in the British National Health Service is politically sensitive and has to be adapted accordingly. It is not like asking respondents' WTP for environmental goods. Experience of using the WTP technique in the British NHS has shown that providers of health care are concerned that including WTP questionnaires will lead to suggestions that they are attempting to privatize the health care system. Such providers have therefore insisted on such a statement being included if WTP is to be used as an instrument to value health care interventions. Inclusion of this statement was also intended to reduce "protest bids", i.e. respondents not answering the question because they have a moral obj,ection to being asked WTP for health care. It is recognized that such a statement may invite respon-
dents to overstate their WTP and that work is therefore needed on the validity of the WTP technique using this statement. However, in the only empirical study testing the "overpledging hypothesis", Bohm (1972) found no evidence of respondents overstating their WTP when they are told that they would not actually have to pay anything, no matter what they said they would be WTP. Bohm looked at respondents' WTP to see a preview of a Swedish television show. He was concerned with numerous aspects of strategic bias, including free-riding and overbidding. Respondents were allocated to one of five possible payment situations: actual payment of the WTP offered by each individual; payment amount proportional to WTP; lottery chooses payment method; fixed payment of 5 Kroner and no payment required. In the first two scenarios freeriding was hypothesized, in the third no strategic behaviour was expected, and in the latter two overbidding was expected. However, Bohm found that respondents were not sensitive to the strategic incentives included in the experiment, and that the WTP amounts revealed in the five scenarios were not statistically significantly at any accepted significant level.
Econometric analysis Logistic regression was used to estimate WTP. To test whether there was a significant difference in WTP across the two arms of the trial (GPM care versus shared care), arm of the trial was also included as an independent variable in the logit regression equation. This took on a value of zero if the woman was randomized to the new style of care and one if she was randomized to the old style of care. If there was a significant difference in the values individuals gave to each type of antenatal care, this would be expected to be reflected in the arm of the trial being a significant predictor of WTP (i.e. having a p value less than or equal to 0.05). Other variables which were hypothesized, a
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priori, to influence WTP for antenatal care were
also included in the logistic regression equation. These variables are shown in Table 2. Inclusion of such independent variables also allows for the testing of the internal validity of the WTP technique, i.e. the extent to which the results are consistent with economic theory, or, more generally, a priori expectations. For example, it was hypothesized, a priori, that the higher the level of satisfaction with previous experience of antenatal care in previous pregnancies, the greater would be WTP for current antenatal care. Similarly, the hypothesis behind inclusion of perceived risk of having problems with own health and babies health during pregnancy was that a greater perceived risk would lead to an increased WTP. It was hypothesized that the higher the income level, the higher would be WTP. There was no a priori hypotheses for inclusion of number of antenatal care appointments to date nor age of respondent. All variables were initially included in the model (the general model). A general to specific regression method was then used, with explanatory variables being excluded from the general model in a stepwise fashion. Using this method, the first variable excluded from the regression equation is the one with the smallest positive or negative correlation with the dependent variable. The F-test for the hypothesis that the coefficient of that variable is zero is then estimated. The variable is dropped from the equation only if the probability associated with the F-test is greater than or equal to 0.1. This variable is then examined and re-entered if the probability associated with the F-test is smaller than 0.05. Having carried out these tests on the first variable, all variables in the equation are again tested for exit and entry according to the above steps. This procedure continues until none of the variables remaining attains exit or entry criteria (Norusis, 1990). Given that a substantial proportion of respondents had no previous experience of antenatal care, and therefore did not provide information on satisfaction with antenatal care, the logistic regression equation was reran without satisfaction as an independent variable. A general and specific equation were again estimated.
In estimating mean WTP, explanatory variables are set equal to their sample means. A linear model was assumed since this is consistent with economic theory (Hanemann, 1984). The upper and lower limits of the integration were taken as zero to plus infinity since it is assumed that if individuals have a negative utility, they would choose not to have treatment. When estimating logistic models goodness of fit measures include the log-likelihoods, the chi-squared (which use the log-likelihood to test the overall significance of the model) and the proportion of correct predictions made by the estimated model compared with the original data. These statistics are all presented for the logistic models estimated in this paper.
RESULTS
Of the 956 questionnaires mailed, 704 were returned, giving a response rate of 74%. Of these respondents 658 (93%) completed the WTP section. This suggests that the WTP question is acceptable to respondents. Table 3 shows the results from both the general and specific models when satisfaction is included as an explanatory variable. In the specific model, all of the variables have the expected signs. The negative sign on the bid variable indicates as we would expect that the higher the bid amount offered to the respondent, the greater the probability that they will say no. The negative coefficient on age implies that older women place a lower value on antenatal care. The positive sign on satisfaction implies that the more satisfied women are with their antenatal care, the more they are willing to pay. This is what we would expect. The positive and significant sign on income provides support for the theoretical validity of the CE WTP technique, with WTP being positively related to income. Average WTP is estimated to be £2703 from the specific model, with no significant difference between the two arms of the trial. This suggests that arm of the trial does not have a significant impact on the valuation given to antenatal care. The number of observations in the initial model is only 312. This is due to the fact that a large num-
Table 2. Variables included in the logit regression model Description o f variable Bid level offered to individual A r m o f the trial (0 = G P M , 1 - shared care) Age o f respondent N u m b e r o f antenatal care appointments had so far during pregnancy Satisfaction with previous experience o f antenatal care (scale of 1 5 where 1 represents very dissatisfied and 5 very satisfied) H o w likely do you think you are to have problems with your health during your pregnancy (scale of 1-5 where I = very likely and 5 = very unlikely) H o w likely do you think you are to have problems with your baby duting your pregnancy (scale of 1-5 where I = very likely and 5 = very unlikely) Income (1 = less than £6000; 2 = £6000-£10,000; 3 = £10,001-£15,000; 4 = £15,001-£20,000; 5 = £20,001£25,000; 6 = £25,001-£30,000; 7 = £30,001-£35,000; 8 = greater than £35,000
Variable name BID ARM AGE NUMAPPT SATIS HEALTHPR PREGPROB INCOME
Willingness tc, p a y in a n t e n a t a l care
377
Table 3. Logit regression results for general and specific model when including satisfaction General model Attribute
Coefficient
Constant BID ARM AGE NUMAPPT SATIS HEALTHPR PREGPROB INCOME n Log-likelihood McFadden R 2 ;(2 Correct predictions Mean WTP (£) 9 5 0 confidence intervals
2.0583 -0.0003 -0.0268 -0.0872 0.0276 0.5342 -0.1264 0.0470 0.4736 312 - 185 O. 14
Specific model P
Coefficient
P
0.0854 0.0005 0.9165 0.0249 0.4899 0.0024 0.4190 0.7918 0.0001 312 - 185
2.0260 -0.0003
0.0494 0.0005
-0.0913
0.0181
0.5245
0.0025
0.4657
0.0001
O. 14
62.16 (0.001) 68% 2548 1701-5183
60.96 (0.001) 68% 2703 1813-5772
ber of respondents had no experience of antenatal 93% of these respondents completing the WTP care previously and therefore could not provide question, suggested that individuals were willing to data on their level of satisfaction with previous ex- complete a CE WTP questionnaire. However, this perience of the service. In the next step the satisfac- high response rate may be partly explained by the tion variable was excluded from the logistic fact that respondents were taking part in a clinical regression analysis to increase the number of obser- trial. The positive and statistically significant coeffivations. Table 4 shows this model. This makes little cients on income and satisfaction were as expected difference to the results, with bid level, age ~,nd and provide support for the internal validity of the income level remaining statistically significant in the technique. same direction as above. Willingness to pay is now The findings suggest that average WTP for ante£2467, and there is again no significant difference in natal care is £2467 (for the model excluding the satWTP between the two arms of the trial. isfaction variable), with no significant difference between the arms of the trial. However, these findings should be interpreted with care for a number DISCUSSION AND CONCLUSION of reasons. Whilst using the CE approach may This paper used the CE WTP technique to esti- make the type of questions easier to answer than mate the value of alternative models of antenatal using the open-ended, payment card or bidding care. One of the main aims was to establish whether technique (and this is ultimately an empirical questhere was a significant difference in the valuation tion), there are still several methodological quesgiven by respondents in the two arms of the trial. tions around the analysis of CE data which need to By using the economic instrument of WTP, the be addressed. These include devising the bid vector study also attempted to take account of such altri- as well as considering the reliability and validity of butes as information, reassurance and knowing the technique. With regard to the bid vector, you have done everything to prevent an abnomaal Duffield and Paterson (1991) carried out sensitivity analysis on CE WTP estimates, using Monte Carlo foetus. The findings provide support for the use of the simulations. They kept total sample size and the bid CE WTP approach. The response rate of 74%, with values fixed, but reallocated the number of individTable 4. Logit regression results for general and specific model when excluding satisfaction General model Attribute Constant BID ARM AGE NUMAPPT HEALTHPR PREGPROB INCOME n Log-likelihood McFadden R z X: Correct predictions Mean WTP (£) 95% confidence intervals
Specific model
Coefficient
P
Coefficient
P
0.2180 -0.0004 0.1569 -0.0459 0.0203 -0.0715 0.0685 0.3139 563 -351 0.10 78.02 (0.0001) 67% 2354 1713-3854
0.7670 0.0001 0.3899 0.0681 0.4755 0.5470 0.6053 0.0001 563 -352 0.10 78.02 (0.0001) 67% 2467 1773-3884
0.4370 -0.0004
0.4593 0.0001
-0.0466
0.0613
0.3097
0.0001
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uals assigned to each bid level. Mean WTP was attributes. This approach has been found to be senfound to be sensitive to such reallocations. Cooper sitive to process and non-clinical outcomes in health and Loomis (1992) looked at the sensitivity of WTP care (Ryan and Farrar, 1994; Ryan, 1995a). The estimates to both the bid vector design alternative authors plan to apply conjoint analysis in future measures of WTP. They concluded that the analysis work looking at alternative models of antenatal of 10 different CE WTP data sets indicated a sensi- care. tivity in estimated WTP to the bid vector design From a policy perspective, the results from the and the method chosen to estimate WTP. costing part of the trial show that GPM care costs Regarding reliability and validity, whilst evidence society statistically significantly less than shared from the environmental literature supports the re- care (Ratcliffe et al., 1996). Clinical outcomes and liability (Jones-Lee et al., 1985; Heberlein, 1986; consumer satisfaction were shown to be the same in Loomis, 1988; Loehman and De, 1992) and validity the two arms of the trial (Tucker et al., 1995), and (Cummings et al., 1986; O'Brien and Viramontes, the results of the WTP study presented here indicate 1994) of the WTP approach, it is not clear at the that there is no difference in strength of consumer moment whether these results are transferable to preference between the two alternative forms of health care. The commodity of health care clearly care. As such, GPM antenatal care would be the differs in many ways from many of the commodities efficient option (compared to shared care) for routo which the technique of CV has been applied in tine antenatal care for low risk pregnant women the environmental literature. Studies are thus throughout Scotland. needed to assess the reliability and validity of the technique within health care. Acknowledgements--This study was funded by the Health Secondly, the insignificance of the arm of the trial Services and Public Health Research Committee of the in the regression equation may reflect the insensitiv- Chief Scientist Office of the Scottish Office Home and ity of the WTP instrument to pick up the import- Health Department (grant number K\OPR\2\2D63), but ance of particular process attributes. In the the views expressed are those of the authors. We thank all the women who completed the questionnaire and Cam environmental literature there has been a big debate Donaldson for comments made on earlier drafts of the about whether CV is sensitive to the size of the pro- paper. gramme being evaluated. For example, Boyle et al. (1994) found that people's WTP to take action to REFERENCES prevent the death of 2000 migratory birds in an oil spill is the same as their WTP to prevent 20,000 and Andreano, R. L. and McCollum, D. W. (1983) A benefit 200,000 such deaths. 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380 APPENDIX A
W T P Question
In this section I am interested in the value you place on antenatal care. One way of finding out how much you value antenatal care is to find out the maximum amount of money you would be willing to pay for the provision of the service if it were not freely available on the NHS. THERE IS NO WAY YOU WOULD HAVE TO PAY FOR THIS SERVICE. THE AIM OF THE QUESTION IS ONLY TO FIND OUT HOW MUCH YOU VALUE ANTENATAL CARE.
22. Would you be willing to pay £10,000 for antenatal care during your pregnancy? Imagine the amount could be paid by extra taxation or through a voluntary donation. Also remember that any contribution made would reduce what you have left to spend on yourself. So would you be willing to pay £10,000' for antenatal care during your pregnancy? Yes
~
No *Bid amount varied from £25 to £10,000