Methodological concerns

Although CV is widely used in the economic valuation of environmental resources, its use has been the subject of considerable criticism (Diamond and Hausman, 1994;

Diamond et al., 1993). At the heart of such criticism lie concerns about the reliability and validity of CV studies. Reliability concerns the degree of replicability of the measurement over time and over different applications (i.e. generalisability), whereas validity pertains to the degree to which a study measures what it sets out to measure.

Validity analyses measure the degree to which biases influence the obtained WTP estimates. The validity of a study can be defined as the extent to which it measures the theoretical construct under investigation (Mitchell and Carson, 1989).

There are two types of validity: content validity and construct validity (Mitchell and Carson, 1989; p190). Content validity refers to the extent to which an estimate takes account of all the issues deemed important for the study. This includes whether the CV survey asks the correct questions in a clear, understandable and appropriate manner so that a valid estimate of WTP is obtained. As such, content validity deals with all aspects of survey design. Content validity is, however, difficult to assess as it depends on the subjective judgment and experience of the person reviewing the study.

Construct validity is concerned with two sub-issues (Mitchell and Carson, 1989).

Convergent validity is the issue of correspondence, or convergence, between measures obtained by different valuation methods. Theoretical validity, refers to the extent to which the findings conform to the theoretical foundation of the study, and to prior expectations. For example, fundamental economic theory requires that statements of willingness-to-pay are restricted by an individual’s ability to pay. Thus, an influence of an indicator of individual income or wealth on stated maximum WTP is expected.

Problems with reliability and validity may arise due to certain biases. The main bias criticism has been centered on the technique’s hypothetical nature. CV is a hypothetical method because it relies on mere statements of preference that need not reflect future behavior well. This hypothetical nature can result in hypothetical bias.

A common concern is that without real resources at stake, the response to willingness to pay (WTP) questions is meaningless. What this means is that there is no budget constraint in a hypothetical market; without a budget constraint, WTP statements are regarded as meaningless (Kolstad, 2000).

Depending on their design, CV questions can be categorized as:

Open-ended: Individuals are asked to state their maximum willingness-to-pay without being shown any amounts beforehand. This approach was the first to be used, but has been subject to much criticism, for example, due to the possibility of obtaining unrealistic responses, reflecting mainly an understatement of actual WTP. (Bateman et al. 1999; Hoehn and Randall, 1987).

Bidding game: In this approach, respondents are faced with several rounds of discrete (yes/no) choice questions if they are willing to pay a certain amount for the proposed change or not. The bidding game is continued until the respondent declines to pay the given amount. Though this was one of the most commonly used methods in the early days, it is rarely used today, however, due to considerable evidence for a starting point bias among several issues (Bateman et al., 1999).

Dichotomous choice (also called close ended and referendum): Here, the respondent is confronted with an amount and has the opportunity to accept or reject to pay the given amount. This elicitation format is thought to simplify the cognitive task faced by respondents as it resembles real market choices. However, due to the limited information that it reveals with regard to an individual’s WTP, it requires large samples if efficient estimates of WTP and the bid function are to be obtained (Bateman et al, 1999). The dichotomous choice format results in higher WTP estimates compared to open-ended and payment card approaches (Bateman et al., 1999).

A variant of the dichotomous choice format is the Double bounded dichotomous choice: In this format, the first WTP question is followed up by another dichotomous WTP question, depending on whether the prior offer was accepted or rejected. This format has gained ground, as it is more efficient than its using simple dichotomous choice counterpart formals because i.e. more information on individual preferences is gathered from each respondent (Bateman et al., 1999).

Payment card: This approach was developed as an improved alternative to the open-ended and bidding game formats. In a payment card elicitation format, the respondent is presented with a range of values on small cardboard cards, and is asked to choose the highest amount he/she would be willing to pay. Respondent WTP is equal to or greater than the value of the chosen card but smaller than the next higher value. This method has the advantage of being user-friendly because respondents can visually scan a list of value intervals quickly (Cameron and Huppert, 1989). The type of information obtained by this method is less diffuse than with the referendum method. In addition to finding that someone's WTP is higher (or less) than a specific value, we can also determine in which range that WTP lies (Bateman et al., 1999;

Mitchell and Carson, 1989). Payment cards have been used extensively in the literature and their main advantage over discrete choice formats is that they allow for more information to be extracted from any given sample. It has been reported that in order to obtain estimates with the same efficiency as the ones from a continuous payment elicitation format, one would have to increase the sample size for a