Elsevier

Social Science & Medicine

Volume 60, Issue 3, February 2005, Pages 525-536
Social Science & Medicine

Do divergences between stated and actual willingness to pay signify the existence of bias in contingent valuation surveys?

https://doi.org/10.1016/j.socscimed.2004.05.023Get rights and content

Abstract

This study's objective is to determine the factors that cause divergences over time (differences) between stated willingness to pay (WTP) and actual WTP (purchase behaviour), and consider whether any divergence signifies the presence of bias in contingent valuation studies.

Stated WTP for insecticide-treated bed-nets (ITNs) was elicited from a random sample of respondents using three question formats in Nigeria. The question formats were the bidding game (BG), binary with follow-up (BWFU) and a novel structured haggling (SH) technique. The sales of the nets and a second survey were conducted 1 month after the first survey. In the second survey, factors that might explain the divergences were built into the questionnaire and these together with socio-economic variables were examined for causes of divergences in WTP. Data were analysed using non-parametric tests, testing of means and cross-tabulations.

There were divergences in WTP in all three question formats: 69.4% in the BG, 78.7% in the BWFU and 48.8% in the SH. The higher the stated WTP, the more likely the divergence between stated and actual WTP. The attitude of the community leaders to the ITNs in the BG (p<0.05), the time respondents had to think about their WTP (p<0.05) and the external information they received about the ITNs in the BWFU (p<0.05) all led to divergences in WTP.

We conclude that there are genuine causes of divergences between stated and actual WTP across the three question formats, and that the lesser the criterion validity score, the more the level of divergence in WTP. Studies that compare stated and actual WTP should explicitly determine the causes of divergences in order to assess the role of bias in the divergences.

Introduction

Studies of the contingent valuation method (CVM) to determine consumers’ willingness to pay (WTP) for goods and services are increasingly being conducted to better understand the method and to aid decision-making. In sub-Saharan Africa and other developing regions, the emphasis has been on using CVM to understand demand and set prices for goods and services (Weaver et al., 1996; Whittington, Matsui-Santana, Freiberger, Van Houtven, & Pattanayak, 2002). In developed countries, the CVM has been predominantly used for generating consumers’ monetary values for goods and services for cost-benefit analysis (Klose, 1999; Birch, Gafni, & O’Brien, 1999). Some authors have found the CVM to be a valid method to determine WTP (Chestnut, Keller, Lambert, & Rowe, 1996; Onwujekwe, Chima, Shu, Nwagbo, & Okonkwo, 2001; Zillich, Blumenschein, Johannesson, & Freeman, 2002; Foreit & Foreit, 2003), while others have questioned the validity of the method (Mitchell & Carson, 1989; Diamond & Hausmann, 1994; Liljas & Blumenschein, 2000; Stavem, 2002).

The major criticism of the validity of CVM has been that stated WTP is a poor indicator of actual WTP (Diamond & Hausmann, 1994). The CVM is based on a hypothetical market in which respondents are not actually required to make the contributions they claim to be willing to pay (Foster, Bateman, & Harley, 1997). This has led to experiments to test the external validity of the method, by comparing stated and actual WTP and ascertaining whether the two diverge systematically (Foster et al., 1997). Most of the studies that have compared stated and actual WTP found divergences between the two (Corso, Hammitt, Graaham, Dieker, & Goldie, 2002; Blumenschein, Johannesson, & Blomquist, 1998; Blumenschein, Johannesson, Yokoyama, & Freeman, 2001; Onwujekwe, 2001; Johannesson, Liljas, & Johansson, 1998, 1999; Cummings, Harrison, & Rustrom (1996), Loomis, Brown, Lucero, & Peterson (1997); Cummings, Harrison, & Rustrom, 1995; Foster et al., 1997; Frykblom, 1997; Neill, Cummings, Ganderton, Harrison, & McGuckin, 1994).

Divergences in WTP could occur because of bias or because of legitimate changes in consumers’ values or circumstances. Distinguishing between the two is essential for better understanding the validity of CVM. At least six potential sources of bias have been identified (Foster et al., 1997; Dickie, Fisher, & Gerkins, 1987; Mitchell & Carson, 1989; Whittington, Briscoe, Mu, & Barron, 1990; Kealey et al., 1988). First, respondents might be influenced by the hypothetical nature of the situation, which might fail to provide sufficient incentive for respondents to reveal their true valuations. Second, the scenario could contain incentives for respondents to misrepresent their true WTP amount. For instance, if subjects perceive that the information could be used for price setting, they might understate their WTP. Third, the scenario may contain implied value cues that influence WTP amounts. Fourth, there may be mis-specification or mis-perception of the scenario, leading the respondent to understand or correctly perceive the characteristics of the good being described by the interviewer. Fifth, the individual may not take the contingent valuation questions seriously and simply respond by giving whatever answer occurs first to them. Finally, people may lack experience with aspects of CVM itself or lack knowledge about the good. Many, but not all, of these can be addressed through careful attention to experimental design.

Alternatively, there may be real changes in respondents’ valuations or circumstances, which affect their behaviour when the good in question is actually offered for sale. Whittington, Smith, Okoroafor, Liu, and McPhail (1992) studying WTP for water services in Nigeria debriefed respondents and found that they had logical reasons for changing their minds and concluded that there was no evidence of strategic bias in their study. Similarly, in determining test–retest reliability of WTP, Whitehead and Hoban (1999) found that there were socio-economic factors that genuinely accounted for the divergence between stated WTP at test and at retest.

In practice, calibration of stated WTP responses has been used to limit divergence between stated and actual WTP, by adjusting for the degree of certainty in a hypothetical yes response (Blumenschein, Johannesson, & Blomquist (1998), Blumenschein, Johannesson, Yokoyama and Freeman (2001)). For example, Johannesson, Liljas, & Johansson (1998), Johannesson et al. (1999) found that “definitely sure” yes responses were significantly lower than actual yes responses, and thus provided a lower bound for the estimate of real WTP. Similarly, Champs, Bishop, Brown, and McCollum (1997) found that hypothetical donations significantly exceeded real donations, but there were no significant differences if only subjects that were “very certain” of their yes response were counted as real yes responses.

One potential approach to improving the reliability and validity of CVM is increasing the realism of the contingent market in terms of specifying the item to be valued, describing the contingent market, explaining the method of payment and selecting a questioning format for eliciting values (Reiling, Boyle, Philips, & Anderson, 1990). Matthews, Rocchi, Wang, and Gafni (2001) posited that mimicking the live environment in CVM could help to improve validity of elicited WTP. Comparing across studies, Kealey et al. (1988) suggested that the discrepancy between hypothetical and actual CVM valuations is smaller when the commodity is more important to the respondent and that improving the realism of the CVM scenario could induce more intention-consistent or truthful behaviour.

This paper presents findings of stated and actual WTP for insecticide-treated bed-nets (ITNs) and explores the causes of divergences between the two. Specifically, it aims to identify whether divergences in WTP can be attributed to changes in socio-economic variables and other factors that influence WTP. Such evidence would support the view that not all divergence between stated and actual WTP is due to bias, and that changes in circumstances and values may be legitimate causes of such divergence. The divergences in WTP elicited with three question formats are examined. One of the question formats was structured haggling (SH), a novel Nigeria-specific technique designed as part of the study to increase the realism of the CVM question formats in the study area. It was hypothesised that a CVM question format that more closely resembled local patterns of bargaining for goods would more closely simulate actual purchasing thought processes and thereby reduce divergence between stated and actual WTP.

Section snippets

The sample

The study was conducted in three villages that have high malaria transmission rates all year namely; Amaetiti, Ahani and Enugu Akwu in Achi, Enugu State, Southeast Nigeria. The people are of “Igbo” ethnic group and Christianity is the major religion, with subsistence farming being their major source of livelihood. Bargaining for goods is the norm in local markets. Fixed prices operate in supermarkets and some shops although mainly in the cities and mostly for select people that live in the

General descriptive statistics

First survey: The usable numbers of questionnaires for the analysis were 261, 267 and 273 for the BG, BWFU and SH technique, respectively. In the three question formats, the majority of respondents were heads of households, middle aged, females, married and farmers. A minority of the respondents had formal education and the average years of formal education were 3.5, 4.0 and 5.1 in BG, BWFU and SH, respectively. A total of 75.1%, 80.5% and 75.5%, respectively, of the respondents in the BG, BWFU

Discussion

In contingent valuation, not all divergent WTP may be due to bias but because of justifiable determinants of demand change as was found in this study. Hence, divergences between stated and actual WTP should be regarded prima facie as an indicator of poor external (criterion) validity. Our results suggest that researchers need to determine why divergences in WTP exist, and to characterise the factors that could cause them. Divergences may be caused by legitimate factors that alter consumers’

Conclusion

There were genuine reasons people changed their minds about their WTP for ITNs, and thus not all WTP divergences could be said to be due to strategic or hypothetical bias. Therefore, “in order to better understand why hypothetical and real CV valuations may differ, it is important to analyse the motivation in individual answers to hypothetical questions and develop a theory of how individuals respond to hypothetical questions” (Blumenschein et al., 1998). These divergences would also inform

Acknowledgements

We are grateful to the Health Policy Unit, LSHTM for funding the writing-up of this paper. This study received financial support from the UNDP/World Bank/WHO Special Programme for Research and Training in Tropical diseases. Kara Hanson and Julia Fox-Rushby are members of the Health Economics and Financing Programme, which is funded by the UK Department for International Development.

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