Eliciting Willingness to Pay without Bias using Follow-up Certainty Statements: Comparisons between Probably/Definitely and a 10-point Certainty Scale

Correction for hypothetical bias using follow up certainty questions often takes one of two forms: (1) two options, “definitely sure” and “probably sure”, or (2) a 10-point scale with 10 very certain. While both have been successful in eliminating hypothetical bias from estimates of WTP by calibrating based on the certainty of yes responses, little is known about the relationship between the two. The purpose of this paper is to compare the two using data from three field experiments in a private good, dichotomous choice format. We compare four types of yes responses that differ in the criterion used to determine if there is sufficient certainty for a hypothetical yes response to be considered a true yes response. We make several comparisons, but focus on determining which values on the 10-point scale give the same estimates of WTP as “definitely sure” hypothetical yeses and real yeses (actual purchases). Values that produce equivalence are near 10 on the certainty scale.     

‘Adopt a Hypothetical Pup’: A Count Data Approach to the Valuation of Wildlife

The willingness to pay (WTP) for a coyote conservation program is estimated using a novel payment-vehicle, based on how many ‚problem’ coyotes respondents would be willing to sponsor for a year. This hypothetical scenario mimics an increasingly popular type of actual market. Data from a phone survey conducted in Prince Edward Island are analyzed using count data models that consider different processes explaining zero responses and the level of positive responses. This is particularly important in the case of coyotes, often regarded as an economic bad. Estimates of WTP per coyote per year around $18–$22 and annual WTP per contributor of about $46–$57 are obtained.      

Hypothetical Bias in Risk Preferences as a Driver of Hypothetical Bias in Willingness to Pay: Experimental Evidence

This study provides new insight on the established issue of elicitation bias in hypothetical choice settings. In particular, using an experiment that elicits real and hypothetical willingness to pay for the protection of otters, we provide evidence that the observed bias is driven by those who are both uncertain about their value and exhibit hypothetical bias in risk attitudes. This finding provides a reason why calibration approaches based on follow-up certainty questions have proven useful, but also suggests a degree of caution in applying the approach. Our results favour combining uncertainty calibration with CVM-X in which hypothetical bias in risk attitudes is also taken account to correct for hypothetical bias in WTP.     

A comparison of empirical models used to infer the willingness to pay in contingent valuation

The essence of the contingent valuation method consists of creating a hypothetical market where respondents are asked about their willingness to pay (WTP) for a non-market good. Different empirical models can be formulated to estimate the expected WTP of a sample of respondents and, then, through aggregation, the social valuation of the good is inferred.This paper outlines the relevance of the distributional assumptions when estimating mean WTP. Several parametric and non-parametric methods are discussed and applied to calculate the existence value of a natural space. Results show that WTP is extremely sensitive to the empirical model used.First version received: November 2002 / Final version received: January 2004     

Preference uncertainty in contingent valuation

In this paper, the results of empirical studies that applied two widely used methods - numerical certainty scale (NCS) and polychotmous choice (PC) - for estimating preference uncertainty adjusted willingness to pay (WTP) in contingent valuation (CV), are summarized. For this review, a number of conclusions are reached. First, there is a lack of consensus about which method is more appropriate for measuring preference uncertainty. Second, although preference uncertainty information has been found useful in detecting the incidence of hypothetical bias in CV studies, a consensus about a standard certainty threshold (or treatment mechanism) at which hypothetical behaviour converges to real behaviour is yet to emerge. Third, insufficient empirical evidence exists about the causal relationship between preference uncertainty scores and the theoretically expected explanatory variables. Finally, the preference uncertainty adjusted PC and NCS models fail to provide a consistent and more efficient welfare estimate compared to the conventional dichotomous choice certainty model.     

Cognitive ability and scale bias in the contingent valuation method

This study investigates whether or not the scale bias found in contingent valuation (CVM) studies on mortality risk reductions is a result of cognitive constraints among respondents. Scale bias refers to insensitivity and non-near-proportionality of the respondents’ willingness to pay (WTP) to the size of the risk reduction. Two hundred Swedish students participated in an experiment in which their cognitive ability was tested before they took part in a CVM-study asking them about their WTP to reduce bus-mortality risk. The results imply that WTP answers from respondents with a higher cognitive ability are less flawed by scale bias.     

A Meta-study Investigating the Sources of Protest Behaviour in Stated Preference Surveys

It is a well-known empirical finding that some percentage of respondents participating in Stated Preference surveys will not give responses that reflect their true preferences. One reason is protest behaviour. If the distribution of protest responses is not independent of respondent or survey characteristics, then simply expelling protesters from surveys can lead to sample selection bias. Furthermore, WTP estimates will not be comparable across surveys. This paper seeks to explore potential causes of protest behaviour through a meta-study based on full datasets from 38 different surveys. The objective of the study is to examine the effect of respondent specific variables as well as survey specific variables on protest behaviour. Our results suggest that some of the differences in WTP typically observed between different demographic groups, different elicitation formats and different question formats might actually be attributed to inherent differences in the propensity to protest. Our results indicate that the propensity for respondents to exhibit protest behaviour when asked a stated preference type valuation question depends on a number of specific factors, respondent specific as well as survey specific—knowledge which could be used in order to reduce protest behaviour.     

Giving Stated Preference Respondents “Time to Think”: Results From Four Countries

Previous studies have found that contingent valuation (CV) respondents who are given overnight to reflect on a CV scenario have 30–40% lower average willingness-to-pay (WTP) than respondents who are interviewed in a single session. This “time to think” (TTT) effect could explain much of the gap between real and hypothetical WTP observed in experimental studies. Yet giving time to think is still rare in binary or multinomial discrete choice studies. We review the literature on increasing survey respondents’ opportunities to reflect on their answers and synthesize results from parallel TTT studies on private vaccine demand in four countries. Across all four countries, we find robust and consistent evidence from both raw data and multivariate models for a TTT effect: giving respondents overnight to think reduced the probability that a respondent said he or she would buy the hypothetical vaccines. Average WTP fell approximately 40%. Respondents with time to think were also more certain of their answers, and a majority said they used the opportunity to consult with their spouse or family. We conclude with a discussion of why researchers might be hesitant to adopt the TTT methodology.     

Willingness-to-pay for urban ecosystem services provision under objective and subjective uncertainty

There is growing concern that failure to acknowledge the risk and uncertainty surrounding ecosystem services (ES) delivery could have adverse effects on support for ES policy intervention in the long run. However, acknowledging risk may reduce support for policy interventions in the short term. In this paper, we sought to determine whether willingness-to-pay (WTP) for urban forest ES in Southampton, UK is affected by objective and subjective uncertainty surrounding ES delivery. We conducted a discrete choice experiment with a split sample design: one with a scenario specifying risky ES outcomes and one where zero risk was implied. Respondents’ subjective certainty surrounding the provision of ES was determined before and after the choice questions. Despite respondents’ risk aversion, introducing an objective likelihood attribute did not reduce WTP compared to the scenario with implied certain ES outcomes. Furthermore, whilst WTP for the overall scheme was found to be adversely affected by the presence of risk around ES outcomes, subjective uncertainty seemed to reduce WTP more than objective probabilities. Our results therefore support the idea that both objective probabilities and subjective uncertainty should be explicitly incorporated in the design of stated preference studies for ES valuation.     

Dealing with preference uncertainty in contingent willingness to pay for a nature protection program: A new approach

In this paper, we propose an alternative preference uncertainty measurement approach where respondents have the option to indicate their willingness to pay (WTP) for a nature protection program either as exact values or intervals from a payment card, depending on whether they are uncertain about their valuation. On the basis of their responses, we then estimate their degree of uncertainty. New within this study is that the respondent's degree of uncertainty is “revealed”, while it is “stated” in those using existing measurement methods. Three statistical models are used to explore the sources of respondent uncertainty. We also present a simple way of calculating the uncertainty adjusted mean WTP, and compare this to the one obtained from an interval regression. Our findings in terms of determinants of preference uncertainty are broadly consistent with a priori expectations. In addition, the uncertainty adjusted mean WTP is quite similar to the one derived from an interval regression. We conclude that our method is promising in accounting for preference uncertainty in WTP answers at little cost to interviewees in terms of time and cognitive effort, on the one hand, and without researcher assumptions regarding the interpretation of degrees of uncertainty reported by respondents, on the other.     

Choosing Between Internet and Mail Survey Modes for Choice Experiment Surveys Considering Non-Market Goods

In a choice experiment setup considering a non-market good, this paper adds to the literature on survey mode effects by providing evidence that internet surveys can be a viable alternative to traditional mail surveys when gathering feedback from a sample of respondents. The case study concerns preferences for protecting different types of landscape from road encroachment when building new motorways in Denmark. Two samples of respondents are surveyed—one by internet and one by mail. The performances of the two samples are compared over six different criteria; response rates, protest responses, demographics, preferences and WTP, estimation precision, and, finally, certainty in choice. Differences are observed for some of these criteria, implying that analysts should be aware that choosing internet over mail could be accompanied by a survey mode effect. However, the observed differences do not translate into significant differences in the unconditional WTP estimates. In most applied economic valuation studies of non-market goods, the main objective is in fact estimation of WTP. Hence, in the present case, the identified survey mode effects do not severely invalidate the applicability and continued use of the internet as a suitable means of collecting data for choice experiment economic valuation of non-market goods.     

Testing the Effectiveness of Certainty Scales,Cheap Talk,and Dissonance-Minimization in Reducing Hypothetical Bias in Contingent Valuation Studies

Stated preference methods such as contingent valuation and choice modeling are subject to various biases that may lead to differences between actual and hypothetical willingness to pay. Cheap talk, follow-up certainty scales, and dissonance minimization are three techniques for reducing this hypothetical bias. Cheap talk and certainty scales have received considerable attention in the literature, but dissonance minimization has not previously been experimentally tested. Using a four-way split sample design involving over 600 subjects, results from an actual referendum on provision of a quasi-public good were compared with three similar but contingent referenda employing the three bias-reducing techniques. Hypothetical bias was again present. Certainty scales, when properly calibrated, and dissonance minimization were found to be most effective in reducing the bias.     

Should all Choices Count? Using the Cut-Offs Approach to Edit Responses in a Choice Experiment

Should we give equal weight to all responses in a choice experiment? Previous Choice Modelling papers have considered the issue of the internal consistency of choices—such as the extent to which strictly-dominated options are chosen. In this paper, a different focus is employed, namely the extent to which people choose options which violate their stated upper or lower limits for the acceptable levels for individual attributes. Since hypothetical over-statement of WTP has been a focus in stated preference studies, we concentrate on violations of stated upper limits of WTP, and explore the effects of “editing” such choices using a variable censoring rule and alternative approaches to re-classifying choices. The empirical case study is a choice experiment on eco-tourists in Rwanda. Our main conclusion is that the suggested approach offers a useful way of imposing consistency on choices, and that editing choices in this manner has an appreciable impact on estimated willingness to pay. However, issues remain with regard to explaining why people apparently violate their stated maximum willingness to pay.     

ECONOMIC VALUATION OF MORTALITY‐RISK REDUCTION: STATED PREFERENCE ESTIMATES FROM THE UNITED STATES AND CANADA

Two internet‐based surveys were conducted with adults aged 35 to 84–885 respondents in the United States and 641 respondents in Canada—to estimate willingness to pay (WTP) for reducing mortality risks through out‐of‐pocket costs for health‐care programs. All respondents were asked a series of choice questions followed by a payment‐card question. Causes of death included cancer and heart attack. Levels of annual mortality‐risk reduction were 1, 2, and 5 in 10,000. Converted to values of statistical life, results were in the range of $4–5 million (2002 U.S. dollars) for the choice‐question results for a 2‐in‐10,000 annual risk reduction for illness‐related mortality. U.S. and Canadian results were similar. The payment‐card results were about 50% lower than the choice‐question results. WTP to reduce mortality risk was essentially the same for cancer and heart attack. The results showed WTP weakly increasing with age, and no evidence of lower WTP for older adults versus middle‐aged adults. (JEL Q51)     

Exchange Rules and the Incentive Compatibility of Choice Experiments

The incentive properties of stated-preference surveys continue to be a central debate in the valuation of public goods. The majority of empirical studies have focused on incentive properties of contingent valuation questions in relation to situations where answers have monetary consequences. This research explores the incentive properties of repeated, attribute-based choice questions when subjects are provided with an explicit connection between choices and outcomes. Two market/provision-rules are investigated: a posted-price market and a plurality-rule vote. These two provision rules are contrasted to treatments in which no provision rule is discussed—subjects are simply asked to choose their preferred alternative. These three hypothetical choice treatments are compared with a binding choice treatment. While none of the public good treatments are theoretically incentive compatible, we include a comparison of hypothetical and binding choices for a private-good that is incentive compatible. The private good experiments indicate that marginal willingness to pay (WTP) estimates from the hypothetical treatment are larger, but not statistically different than corresponding estimates in the binding choice treatment. Results for the public good experiments indicate that marginal WTP estimates from the hypothetical treatments are much larger, and statistically different than corresponding estimates in the binding choice treatment. The bias is largest when no provision rule is discussed. The bias is reduced with the inclusion of a provision rule, but surprisingly, there was no difference across provision rule treatments. Overall, our results indicate that choice experiments involving a public good should include a provision rule to reduce bias, but the resulting marginal WTP estimates may still be more biased, on average, than those arising from contingent valuation survey formats.     

Donation Payment Mechanisms and Contingent Valuation: An Empirical Study of Hypothetical Bias

Donation payment mechanisms are well suited forsome contingent valuation studies. In aneffort to better understand the discrepancythat has been consistently found between actualand hypothetical donations, we investigate anapproach to estimating actual willingness todonate using contingent donations with afollow-up question in which respondents ratethe level of certainty about their response tothe contingent donation question. The approachallows us to estimate the magnitude of thehypothetical bias and identify the respondentsresponsible for the bias. Identification ofthe respondents responsible for thehypothetical bias is the first step towarddeveloping an understanding of the causes andpossible remedies. In this study we find thatmost of the respondents (80%) to thecontingent donation question provide a responseconsistent with how we predict they wouldrespond in an actual donation situation.     

Multivariate Mixed Models for Open-Ended Contingent Valuation Data: Willingness To Pay For Conservation of Monk Seals

Although dichotomous choice (DC) contingent valuation (CV) has been recommended by the US NOAA 'blue-ribbon' panel for large-scale contingent valuation studies, useful information can still be obtained from smaller, open-ended (OE) studies, often undertaken as a precursor to a DC survey. The CV study considered here was carried out in Greece and looked at willingness-to-pay (WTP) for protecting the Mediterranean monk seal (Monachus-monachus) in the Aegean area. This is the most endangered seal in the world, and the application of the CV methodology was the first such application in Greece. The OE data consist of two responses: first, a binary response detailing whether or not respondents were in principle prepared to pay for the protection of this seal; secondly, those respondents who answered 'yes' to the first question were then asked to state their maximum WTP for such protection. A multivariate binomial – log-normal mixture model is used to develop a bid function including explanatory variables such as income, sex, age and education. Such a modelling approach provides an alternative to more commonplace tobit estimation. However, the model is extended to include further information which was collected on:(a) an increased WTP amount given in response to information that the initial WTP amount may not be enough to prevent the extinction of the seal;(b) respondents were asked to divide their final WTP amount between use, option and existence values, the latter requiring a multivariate model with four binary and four continuous responses per individual in the same model.The discussion focuses on the methodological issues raised with some comment on the substantive interpretation of results.