Willingness to pay: a feasible method for assessing treatment benefits in epilepsy?

Transcription

1 Seizure 1999; 8: Article No. seiz , available online at on Willingness to pay: a feasible method for assessing treatment benefits in epilepsy? Knut Stavem HELTEF Foundation for Health Services Research and Medical Department, Central Hospital of Akershus, N-1474 Nordbyhagen, Norway Correspondence to: K. Stavem, HELTEF Foundation for Health Services Research, Central Hospital of Akershus, N-1474 Nordbyhagen, Norway Contingent valuation using willingness to pay (WTP) is one of the methods available for assessing the value of a new technology or treatment for a disease in monetary terms. Experience with this method is lacking in epilepsy. The objectives of this study were to assess the acceptability of the WTP method in epilepsy, the level of the responses, and to investigate its validity by comparison with other non-monetary preference measures. Among 397 patients with epilepsy responding to a comprehensive questionnaire, 82 were randomly selected for an interview. They were asked about their WTP for an imaginary new technology which could permanently cure their epilepsy. Fifty-nine patients participated and 57 completed the interview (32 women; mean age 44 years), the majority with well-controlled epilepsy. The patients indicated a median WTP of Norwegian Kroner (NOK) 150, 000 (USD 20,000; GBP 11, 800), interquartile range NOK 50, , 000 (USD 6, , 667; GBP 3, , 559) for this cure. Non-response was low, indicating high acceptability of this method. There was little association between WTP and other preference measures; the Spearman rank correlation coefficient was 0.09 and 0.12 with time trade-off and standard gamble respectively, questioning the validity of this method. Key words: willingness to pay; epilepsy; health economics; evaluation; contingent valuation. Introduction Several cost-of-illness studies of epilepsy have recently been presented 1 7. Other types of economic appraisal studies of epilepsy or epilepsy treatment are rare The existing cost-of-illness studies generally rely on prevalence or incidence rates and different models for assessing direct and indirect costs. They consider direct treatment costs of epilepsy and when indirect costs are estimated, under-employment, excess mortality and sometimes transfer payments are aggregated. Patient preferences and the monetary values for the suffering of the individuals and their relatives are not included. These studies use a human capital approach 11 for evaluating quality of life. In addition estimates of benefits of treatment or values of health states are required in monetary terms for cost benefit analyses, perhaps the most comprehensive of economic appraisal methods 8. In health economics an alternative method for estimating the financial burden of disease, contingent valuation, has attracted increased attention during the last knut.stavem@klinmed.uio.no few years In contingent valuation, individuals respond to theoretical scenarios or options by assigning monetary values to prevention, treatment outcomes or health states 15. Frequently, the subjects are asked about the maximum amount they are willing to pay for a specified theoretical service or goods 15. Willingness to pay (WTP) can be assessed directly by asking clients (expressed preferences), or indirectly by observing their behaviour (revealed preferences). Factors other than health or health outcomes might also be important in the provision of health care, indicating that we should consider all benefits to patients 19, i.e. including the benefits and costs that are most difficult to quantify in monetary terms. Stated preference is one method that can enable us to consider all such benefits 19. The WTP technique has been extensively used to measure preferences in transport and environmental economics Applications of the WTP method in health care have previously been reviewed 13, 21,however, the technique has not been applied to epilepsy 8. The objectives of this study were: (1) to assess the acceptability of the WTP method in patients with /99/ $12.00/0 c 1999 British Epilepsy Association

2 Willingness to pay 15 epilepsy, by asking about the WTP for a hypothetical permanent cure for their disease; (2) to estimate the overall WTP for this hypothetical cure; and (3) to assess the validity of the method by studying the association of overall WTP with other non-monetary outcome measures. TTO and WTP methods. All interviews were conducted by the same interviewer. The scenarios used in the three interview-based methods were phrased similarly to ensure equal contexts, and they were always administered in the following order during the interview: TTO, SG, WTP. Materials and methods Subjects All patients aged years, who had been admitted to the Central Hospital of Akershus or attended the hospital s outpatient clinic for epilepsy (ICD 9, code 345) during a period of seven years ( ), received a comprehensive questionnaire by mail. The diagnosis of epilepsy was confirmed by review of the medical record in 696 patients, using standard criteria. These patients were assumed to be representative for the population of epilepsy patients in the hospital s catchment area of inhabitants, as there are no neurologists in private practice in the area, and general practitioners would generally rely on specialist services at the hospital. From the 397 respondents to this questionnaire, 82 patients were randomly selected for this study, and 57 completed the study. All respondents were assigned a number, and random numbers were generated through the random number function of a spreadsheet. Assessment of preferences Utility can be defined as a cardinal measure of the preference for, or desirability of, a specified level of health states or a specific health outcome 23. This is represented on a utility scale, typically an interval or ratio scale, defined by two anchor states or outcomes and their scores, on which utilities are measured. The anchor states are often defined by full health = 1.0 and death = Different procedures can be used for generating preferences. Standard gamble (SG) is the classic technique for measuring cardinal preferences of different health outcomes under conditions of uncertainty 24. A commonly used substitute for use in health care is the time trade-off technique (TTO) 25. Preferences can also be generated using rating scales, multi-attribute scales, equivalence techniques or WTP Details of the utility assessment process in this project has previously been reported 29, only a brief description is given below. The participating patients in this study received a self-administered questionnaire including the 15D instrument and the EuroQol visual analog scale 33, and within 2 days they were interviewed using the SG, Willingness to pay The WTP for a hypothetical cure for epilepsy was assessed using an interative bidding technique. The starting point was decided by the respondent s answer to an open-ended question; however, if this was difficult for the respondent, the interviewer arbitrarily selected a starting point within a range on a payment card. The intervals in the bidding process were determined by the amounts on the payment card. The WTP question used was adapted from a previous study 34. The hypothetical new technology was presented as a new drug. An English translation of the question used appears in the Appendix. Norwegian Kroner (NOK) were converted to U.S. dollars (USD) using an exchange rate of USD = 7.50 NOK, and to Pound Sterling (GBP) at GBP = NOK. Time trade-off The TTO questions were adopted from another study 35, using a 10 year perspective and a slide-board as a visual aid. Patients were given a choice between two outcomes: (1) living 10 years in their present condition or (2) x years in full health, where x was varied until the patient was indifferent to the two alternatives. The preference weight for the actual health state was determined as x/10. At the beginning of the TTO interview the patients initially scored two theoretical conditions as a warming-up, before proceeding to their own disease state. Standard gamble SG was used for deriving von Neumann Morgenstern utilities, a concept for describing individual preferences in terms of expected utility, that can be treated as numeric quantities 24. The patients faced a choice between two alternatives: (1) living 10 years in their present condition, or (2) participating in a gamble with a probability p of becoming healthy for 10 years and a probability 1 p of immediate death. Then p was varied until the patient was indifferent to the two alternatives. The preference assigned to their own condition was p at the point of indifference. SG was carried out using a 10 year perspective 36, but without a visual aid.

3 16 K. Stavem Table 1: Analysis of respondents with epilepsy at various stages. Total sample Respondents Final interviewees (n = 696) (n = 397) (n = 57) Mean age (years) Gender (%female) Mean duration of epilepsy (years) 17.8 a 16.9 b % with seizures last year % using epilepsy medication % working or full time education a n = 272. b n = 36. EuroQol visual analog scale The EuroQol questionnaire has been developed by a European group as a standard non-disease-specific instrument for describing and evaluating quality of life 33. It is a descriptive classification system consisting of five items, each with three levels presently called EQ 5D. In addition, it contains a vertical visual analog scale of the thermometer type for assessing global health states today from worst imaginable to best imaginable health state (0 100). This study used a 10 cm version of this rating scale, here called the EuroQol visual analog scale (EuroQol VAS). 15D The 15D questionnaire is a self-administered generic profile measure with 15 items, each with five levels. It can be used as a multi-attribute utility measure, with aggregation of the responses to an overall score on a 0 1 scale, using an additive model with relative weights derived from a Finnish general population. 15D was developed in Finland and validated in a large Finnish population The questionnaire was translated independently by two physicians, from an English version into Norwegian, who discussed the translations before agreeing on a consensus version. The Norwegian version has been compared with the original Finnish version, and the translation has been found satisfactory. The use of 15D in Norwegian patients with epilepsy has recently been reported D is being used in numerous evaluation studies in Finland, Norway, Sweden, the United Kingdom, the Czech Republic, Australia, Japan and Israel. An English version of the instrument has previously been published 30. Sociodemographic variables Information about household income, number of children, and the patient s view of their own life expectancy was collected. Data analysis Non-parametric methods were used throughout the study because of asymmetric distributions for WTP and for most quality of life variables. For comparison of respondents to the first questionnaire with final interviewees, the Mann Whitney U test was used. Correlations between WTP and other preference measures were assessed using Spearman rank correlation coefficients. High correlations would support construct validity of the WTP method. Summary statistics are reported for preference measures using the median and interquartile range. A multiple linear regression model was used to identify determinants that could explain the observed variability in WTP; using age, gender, having had seizures in the last year, household income, number of children, self-reported life expectancy in years, and selfperceived health states (using 15D score) as independent variables. For statistical analysis (forward stepwise multiple linear regression and rank correlations) the Statistical Package for the Social Sciences (SPSS) used (SPSS Inc., Chicago, IL, U.S.A.). A level of 5% was chosen for statistical significance. The study was approved by the regional medical ethics review committee. Results Fifty-seven patients (32 women), mean age 44 years (SD 11.7), completed the study. In addition, two patients were interviewed and excluded for lack of comprehension during the interview. Among the patients completing the interviews there were more women, who were younger, used less medication and had experienced fewer seizures during the last year than the respondents to the initial questionnaire, although the differences between questionnaire respondents and interviewees were not statistically significant (Table 1). The patients in the study generally had well-controlled epilepsy, only 25% had experienced seizures during the last year. They scored highly on the various preference measures (Table 2), as expected with well-controlled epilepsy.

4 Willingness to pay Frequency WTP (in Norwegian kroner) Fig. 1: Willingness to pay for a permanent cure for epilepsy. A normal curve is superimposed, centered on the mean. Table 2: Preference values for the patients own condition at time of study (n = 57). 25th 75th Median Percentile EuroQol VAS score Standard gamble score Time trade-off score D score (n = 55) Table 3: Willingness to pay for a permanent cure for epilepsy. 25th 75th Median Percentile WTP (NOK) 150, , , 000 (USD) 20, 000 6, , 667 (GBP) 11, 800 3, , 559 WTP/annual household income Table 4: Spearman rank correlation coefficients (ρ) between WTP and income adjusted WTP (WTP/Income a ) and score on quality of life measures (n = 57). Measure WTP WTP/Income a Standard gamble Time trade-off D (n = 55) EuroQol VAS a Household income. The interviewees stated a median WTP of NOK (USD ), range NOK (USD ) for a hypothetical cure for epilepsy without side-effects; as a proportion of their annual household income, the median was 0.47 (Table 3). The distribution of the responses to the WTP question was skewed, as illustrated in Fig. 1. There was no significant difference in WTP or WTP as a proportion of household income between patients with or without seizures during the last year. In the multiple linear regression model neither age, gender, number of children, life expectancy, having had seizures in the last year, nor self-assessed health states could explain the variability in WTP. Only household income (HI) contributed significantly, explaining only 13% of the variability in WTP. The regression equation (amounts in NOK): WTP = HI. When using a logarithmic transformation of WTP as the dependent variable, household income explained only 11% of the variation in log WTP. There was little or no association between WTP and scores on the utility instruments, as shown by the low Spearman rank correlation coefficient, with all correlation coefficients below 0.20 in absolute value (Table 4). Discussion People with well-controlled epilepsy had few problems responding to the questions about WTP, indicated by only two patients being excluded for lack of comprehension. Hence, the acceptability of the method was good. This could possibly be attributed to the use of face-to-face interviews in this study, in contrast to most other studies of WTP in health care 37. Whether the level of WTP is reasonable and representative for a population of well-controlled epileptic

5 18 K. Stavem patients cannot be ascertained, as there are no figures from similar studies of epileptic patients for comparison. There were, however, wide individual variations in WTP. For comparison purposes, in a similar study of 247 patients with rheumatoid arthritis, patients would pay 22% (mean) of their total monthly household income on a regular basis to get rid of their disease 12, and 5% of monthly income in patients with chronic obstructive lung disease 16, versus a median one-off payment of 47% of one year s household income in this study. A controversial issue is to what extent WTP can be used as measure of the burden of disease, i.e. the validity of the method. If utility or HRQL measures are used as a yardstick, the low associations with WTP in this study questions the validity of the WTP method. However, the correlation between SG and WTP was smaller in this study than previously found for chronic pulmonary obstructive disease patients 16. A recent study of students evaluating theoretical health states found that WTP was not satisfactorily comparable with other trade-off methods 38, suggesting that this method might be unfeasible unless it is possible to improve the operationalization of WTP. In the present study the task in the contingent valuation question was specified as a permanent cure for epilepsy, which might be interpreted as getting rid of not only seizures, but all aspects of disease including the stigma of epilepsy. The finding of no difference in WTP between patients with and without seizures during the last year, and the poor association with utility measures, and the poor association with utility measures, suggests that WTP captures other aspects of patients values than what is quantified in the dimensions of utility assessment and seizure frequency. There are several limitations to this study. The analysis is based on data from a small group of patients with mostly well-controlled epilepsy, and one should be careful about generalizing to other groups. The answers could be biased in different ways. The WTP question might be considered unrealistic and the respondents might lack knowledge of the option under evaluation 15, as the putative wonder treatment does not actually exist. However, this is a general problem in contingent valuation and not specific for this study. In alternative methods for assessing personal preferences, e.g. SG and TTO, similar theoretical scenarios are used. In Norway the public health care services pay for all hospital care, and the patients only pay a small deductible fee for outpatient care. Hence, the context described is unfamiliar and might cause a bias. This could mean an understatement of WTP 22 and also be close to a free rider problem, as the patients might think that these services might be provided by society anyway. Using a different payment vehicle, e.g. raising funds through increased taxes, might have represented a more realistic alternative. On the other hand, since WTP is a hypothetical answer to a hypothetical question and no payment is involved, this could lead to an overstatement of the individual s WTP 22. The bidding technique used here could lead to a starting point bias 13 ; however, in order to minimize this the starting point was, as often as possible, decided by the patient in an open-ended question. The patients might, in practice, face a budget constraint that was not explicitly stated in the question: nor was the possibility of borrowing for financing the hypothetical treatment. Some of the patients interviewed were on a disability pension. Assuming they were restored to full health, they might be able to resume work and could afford to repay a loan. Their WTP might include discounted future increased income. In future applications of WTP, the scenarios and the wording in the questions should be planned very carefully in order to minimize possible biases. The WTP methodology might be useful in economic analysis of specific treatments for epilepsy, e.g. new pharmaceuticals or new operative techniques. The lack of association with other valuation methods, however, questions the validity of the method, and underscores that results of economic analyses are very sensitive to the choice of valuation method. References 1. }Silfvenius, H. Economic costs of epilepsy treatment benefits. Acta Neurologica Scandinavica 1988; 117 (Suppl.): }Gessner, U., Sagmeister, M. and Horisberger, B. The cost of epilepsy in Switzerland. International Journal of Health Sciences 1993; 4: }Cockerell, O. C., Hart, Y. M., Sander, J. W. and Shorvon, S. D. The cost of epilepsy in the United Kingdom: an estimation based on the results of two population-based studies. Epilepsy Research 1994; 18: }Beran, R. G. and Banks, G. 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We are interested in the value you place on a new drug for your disease. One way of doing this is to ask you how much you would theoretically be willing to pay for this drug. This drug instantly cures your disease, has no side-effects, is not harmful, and there is no risk of complications or death. You only need to take the drug once, and it works instantly. What is the maximum amount of money you would be prepared to pay for this drug?