A comparison of two methods for eliciting contingent valuations of colorectal cancer screening

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Abstract

Willingness-to-pay (WTP) is being used increasingly in health technology assessment, although a number of methodological issues remain unresolved. Using data obtained from a randomised questionnaire survey, we investigated the metrical properties of two WTP formats, the open-ended question versus the payment scale, in the context of screening for colorectal cancer. Approximately, 2800 responses were analysed. Household income, attitudes toward health promotion and personal risk perceptions were the principal determinants of the nature and value of response. In comparison with the open-ended format, the payment scale achieved a higher completion rate and generated higher valuations. We believe that a framing effect is the most plausible explanation for these differences in performance. In contrast to previous findings, we do not find subjects’ perceptions of the resource cost of interventions to be a convincing explanation for either their WTP values or inconsistencies between values and preferences. Although a proportion of respondents protested at the notion of valuation, the majority offer positive valuations, although typically of a lower value that non-protesters.

Introduction

Recent reviews and surveys of the use of the contingent valuation (CV) method in health technology assessment attest to the technique’s increasing popularity (Olsen, 1997, Diener et al., 1998, Blumenschein and Johannesson, 1999, Klose, 1999, Smith, 2000). The method was originally developed by environmental economists, in response to a need to elicit public valuations for prospective changes in complex, intangible and non-traded goods, such as the destruction of attractive landscapes and the creation or loss of wildlife habitats. CV poses a hypothetical question, via interview or questionnaire in a variety of possible formats, in order to gauge the minimum amount individuals would be willing to accept in compensation, were a pre-specified adverse change to be imposed upon them. For a potentially beneficial change, CV searches for the maximum amount individuals would be willing to pay, to ensure that the prospective change took place. In theoretical terms, the technique is seeking to identify the compensating variations required to maintain each individual at the same utility level, before and after the prospective change, and thereby to estimate the impact of the change on aggregate utility.

The objective of most published studies has been the estimation of CVs in specific medical circumstances, such as maternity care (Ryan et al., 1997), cervical screening (Wordsworth et al., 2001), or treatments for cancer (Ortega et al., 1998), obesity (Narbro and Sjostrom, 2000), incontinence (Johannesson et al., 1997) and angina (Kartman et al., 1996). However, and as all the survey papers are at pains to point out, the CV technique in health care remains essentially unproven. Many issues are unresolved, and these have been signalled by the relatively small number of experimental studies which have addressed themselves specifically to methodological development, as opposed to estimation. Accordingly, “much research on the contingent valuation methodology in health care needs to be performed before results from CV studies can be used with confidence in health care decision making. This is especially true concerning methodological analysis and testing” (Klose, 1999, p. 117).

In this paper, we report the results of a CV study eliciting willingness-to-pay (WTP) for two different screening tests for colorectal cancer. The study was designed explicitly to enable us to explore a number of issues raised in the methodology literature, and therefore incorporates features not necessarily relevant for WTP studies concerned solely with estimation. We used two different WTP formats, with a view to comparing their performances. Beyond this, our elicitation of WTP values was prompted less by a desire to identify the values per se, and more by an interest in discovering the ostensible reasons behind valuation and in exploring the relationships between reasons and values. The existing literature alerted us to the likely presence of confounding variables in explaining WTP, such as gender, age, income and risk perceptions, making us aware of the need for a large sample for analysis. As a result, with a sample size approaching 2800 responses, this was one of the largest CV studies ever undertaken in the health care field.

In the following section, we review briefly the methodological controversies surrounding WTP, and indicate those which were under specific investigation in this study. Thereafter, we describe the study itself, present the results and develop explanatory models of WTP. Finally, we discuss the implications of our findings in relation to those of earlier researchers, with a view to establishing whether they are consistent both with previous results and the explanations previously offered.

Section snippets

Methodological controversies

Perhaps the most fundamental methodological question for CV is: if people are providing answers to a hypothetical question, to what degree can their answers be interpreted as real, in the sense of providing an appropriate estimate of each individual’s compensating variation (Liljas and Blumenschein, 2000)? Relatively little research into the motives behind providing specific WTP responses appears to have been undertaken (Clark et al., 2000), although income and affordability considerations have

Screening for colorectal cancer

Colorectal cancer is the second leading cause of cancer death in North America and western Europe (Lieberman and Sleisenger, 1996). The disease appears to follow a well-behaved, staged, progression and this, coupled with recent developments in diagnostic technology, have made it a prime candidate for mass population screening (Mulcahy et al., 1997). Whilst a variety of screening protocols are available for general use in an asymptomatic population (Young et al., 1996, Lieberman, 1998), much of

Analysis

Our intention was to analyse the data using regression analysis, and this necessitated a degree of data re-coding, to create either sets of dummy variables (as in the case of household income, for example) or dichotomous variables (as in the case of risk perception). In addition, one construction was undertaken. Evidence suggests that attitudes towards colorectal cancer screening are influenced by subjects’ orientations towards health education and health promotion more generally. This has been

Discussion

Three classes of characteristics of subjects appear repeatedly in all the models estimated. Subjects are more likely to provide a response to the WTP question, less likely to offer a zero value, and provide higher positive WTP values, as income increases, as attitudes towards disease prevention generally become more positive and as perceptions of risk or concern relating to colorectal cancer are heightened. Such results would be predicted intuitively, which provides some degree of confidence in

Acknowledgements

The study forms part of the evaluation of the UK Flexi-Scope trial, funded by the UK Medical Research Council. The authors gratefully acknowledge the assistance of members of the Trent Collaborative Research Network in obtaining the primary data. They are also appreciative of helpful comments from the anonymous referees.

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