Stated preference surveys contingent valuation (CV) and stated choice (SC)--are typically administered to respondents in the course of one phone or in-person interview. Though the format differs, these surveys ask respondents about their willingness to trade income for some environmental or health improvement that is not traded in a market. Because one is not often asked this type of question in everyday life, the answers do not necessarily come easily or reflexively. As every salesperson knows, people often change their minds when they are given overnight to think about a decision and discuss it with others. Despite this, only a few stated preference researchers have explored the effect of giving respondents time to consider their answers. Whittington et al. (1992) and Lauria et al. (1999) gave respondents overnight to think about their answers to a CV survey, but no similar research has been done for SC surveys, which are growing in popularity and are typically more cognitively difficult for respondents to complete than CV surveys.
We hope to fill this gap in the literature. We use a split-sample experiment to explore the effect of giving respondents time to think about their answers in an in-person SC survey of individuals' demand for cholera and typhoid vaccines in Hue, Vietnam. In addition, we analyze the data using Train and Sonnier's (2003) state-of-the-art mixed logit/hierarchical Bayes (MLHB) estimating procedure. Using Monte Carlo--Markov chain numerical methods, Train and Sonnier's approach avoids several strong assumptions typically employed in estimating qualitative response data. It allows the researcher to model taste parameters that (1) vary among respondents, (2) are correlated, and (3) vary according to distributions other than the normal distribution. This research is one of the first applications of this procedure, and the first using data from a developing country.
We examine two principal questions. First, does giving respondents time to think increase the quality of responses; that is, does it reduce the number of responses that violate utility theory (i.e., internal validity tests)? Second, do respondents who were given time to think give us different answers than those who complete the interview in one session? In particular, does giving respondents extra time affect their willingness to pay (WTP) for vaccines?
We find that respondents who were given time to think failed internal validity tests less frequently, although the number of failures in both subsamples was surprisingly low. Respondents with time to think had lower average WTP for the vaccines than respondents without. We also find that respondents with time to think were more sensitive to the price of the vaccine and to the levels of the two other vaccine attributes (effectiveness in protecting against the disease and the duration of protection), though this difference in taste parameters may be due to differences in variance (scale).
The next section explains why we used stated preference techniques for this application and reviews the literature both on measuring internal validity failures in SC studies and on the effect of giving respondents time to think. The third section discusses our research design, and the fourth introduces our data analysis plan and discusses the advantages of the MLHB estimating procedure. The fifth section briefly describes the study site. The sixth section presents our results, and the final section concludes with a discussion of the results.
Using Stated Preference Methods to Measure Vaccine Demand
Though not the primary focus of this article, the overall objective of our research in Hue was to estimate demand for new-generation vaccines against cholera and typhoid fever. We used stated preference techniques (CV, SC) because although vaccines for both cholera and typhoid fever exist, they are not widely available to households in Hue. …