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Payment certainty in discrete choice contingent valuation responses: results from a field validity test.

1. Introduction

Markets do not exist to provide the information necessary for conducting benefit-cost analyses in many public policy decision-making situations. When desired estimates of benefits or costs "are not manifest in the market" (Arrow 1999, p. vi), economists have increasingly turned to contingent valuation surveys to elicit the values that individuals would place on public goods and externalities (Mitchell and Carson 1989; Cropper and Oates 1992; Deacon et al. 1998).

Although discrete choice, take-it-or-leave-it methods of eliciting preferences have gained favor on theoretical grounds (Arrow et al. 1993; Carson, Groves, and Machina 1999) and realism (Hanemann 1994), accumulated evidence from a number of laboratory and field contingent valuation validity studies suggests that these methods overstate actual willingness to pay (WTP) for private and public goods (e.g., Cummings, Harrison, and Rustrom 1995; Brown et al. 1996; Cummings et al. 1997; Balistreri et al. 2001; Champ and Bishop 2001). That is, respondents are more likely to say "yes" to hypothetical commitments than actual commitments, reflecting "hypothetical bias" and the need for "calibrating" contingent valuation responses (Harrison 2002).

Two recent papers offer possible methods for calibrating hypothetical discrete-choice responses by considering payment certainty levels reported by respondents. In what we term the "follow-up certainty question" (FCQ) method, Champ et al. (1997) ask "yes" dichotomous choice respondents to indicate how certain they are, on a scale from 1 ("very uncertain") to 10 ("very certain"), that they would pay the stated dollar amount if the program were actually offered. Separate WTP functions are estimated for each certainty level. Welsh and Poe (1998) instead adopt a "multiple-bounded discrete choice" (MBDC) approach that directly incorporates certainty levels through a two-dimensional decision matrix: One dimension specifies dollar amounts that individuals would be required to pay on implementation of the policy, and the second dimension allows individuals to express their level of voting certainty through "definitely no," "probably no," "not sure," "probably yes," and "definitely yes" response options. A multiple-bo unded logit model is used to estimate separate WTP functions for each certainty level.

In this paper, we use a field validity test of contributions to a green electricity pricing program to further explore these methods and address several validity issues. First, using actual sign-up data as a criterion, we derive "optimal" correction strategies for the two methods. Previous laboratory research on private goods suggests that "yes" hypothetical dichotomous choice responses from those who are "definitely sure" (Blumenschein et al. 1998) or at least "probably sure" (Johannesson et al. 1999) closely predict actual purchase decisions. Johannesson, Liljas, and Johansson (1998) find that respondents who are "absolutely sure" of their decision provide a conservative estimate of real purchases. These laboratory results are replicated in public goods contingent valuation field validity research using FCQ methods, suggesting that models that only use "yes" responses with certainty values on a 1-to-10 scale of "7 and higher" (Ethier et al. 2000), "8 and higher" (Champ and Bishop 2001), or "10" (Champ et al . 1997) best predict actual contributions. We are the first to provide correction strategies for the MBDC approach.

Second, we examine if the experimental "classroom" results reported in Welsh and Poe can be replicated in the field. In that paper, the authors compare estimated logistic response distributions from dichotomous choice questions and MBDC "not sure" responses and find that they are not statistically different. This suggests that respondents who are uncertain of their values will tend to "yea-say" when asked a single dichotomous choice question, a result that has been replicated elsewhere (e.g., Ready, Navrud, and Dubourg 2001).

Finally, in an examination of convergent validity, we compare the MBDC and FCQ methods. Specifically, we compare mean WTP, hypothetical participation rates at $6 (the actual offer price for the program), and the underlying WTP distributions estimated from various models based on the two methods, using both parametric and nonparametric estimation techniques. Conceptually, the FCQ and MBDC methods offer alternative approaches to account for respondent uncertainty in modeling contingent valuation questions. The primary difference between approaches is that the MBDC framework incorporates the certainty correction directly into the discrete choice decision framework, whereas the FCQ method can be regarded as an ex post adjustment to the dichotomous choice response. Although these questions seek the same type of information--how certain an individual is that he or she would actually pay a specified dollar amount--tests of procedural invariance have not been conducted in either the field or the laboratory.

2. Certainty Corrections within the Discrete Choice Framework

The questioning approaches examined in this paper build on previous research indicating that contingent valuation respondents may have a distribution or range of possible WTP values rather than a single point estimate. Here we use the term "certainty" in the same sense as that in Opaluch and Segerson (1989); Dubourg, Jones-Lee, and Loomes (1994); and Ready, Whitehead, and Blomquist (1995). In this framework, when the referendum dollar threshold falls at or below the lower end of the individual's range of WTP values, then the respondent is likely to be very certain that he or she would vote in favor of the referendum. At very high amounts, the respondent might be very certain of voting against the referendum. At intermediate amounts, the respondent is less certain of how he or she actually would vote, with the level of payment certainty being inversely related to the dollar amount.

Dichotomous Choice with FCQ

Response certainty in the FCQ framework is incorporated as follows. Individuals first respond to a standard dichotomous choice (DC) question. For "yes" respondents, a follow-up question is asked:

So you think that you would sign up. We would like to know how sure you are of that. On a scale from "1" to "10," where "1" is "very uncertain" and "10" is "very certain," how certain are you that you would sign up and pay the extra $6 a month if the program were actually offered?

Respondents are asked to circle a response on the 1-to-l0 scale. As empirical evidence suggests that respondents who are uncertain about their willingness to pay tend to respond "yes" (Champ et al. 1997; Welsh and Poe 1998; Champ and Bishop 2001), a follow-up question is not asked of "no" respondents. Modeling of this approach follows well-known DC procedures in which "yes" responses are recoded for each level of certainty and separate WTP functions are estimated. For instance, one can code all responses of, say, 7 and higher as "yes" and all other responses as "no" and then employ standard DC modeling techniques.


The MBDC approach contains elements of and builds on both the payment card (PC) and DC approaches widely used in contingent valuation studies. In a PC question, respondents are presented with several dollar values and asked to circle the maximum value they would be willing to pay. However, rather than circling a single value or interval as an indication of maximum WTP for the referendum, the MBDC approach provides a "polychotomous choice" response option including, say, "definitely no," "probably no," "not sure," "probably yes," and "definitely yes." The respondent then chooses a response option for each of the dollar amounts. In this manner, the context of the good-to-cost trade-off is expanded beyond traditional DC or PC questions by including additional dollar amounts and the likelihood of voting yes, respectively. In some sense, the MBDC model might be thought of as a general framework from which the DC and the PC techniques can be derived as special cases.

Analysis of WTP data collected using the MBDC technique is conducted using a multiple-bounded generalization of single- and double-bounded DC models in which the sequence of proposed dollar values divides the real number line into intervals (Harpman and Welsh 1999). An individual's response pattern reveals the interval that contains his or her WTP at a given level of certainty. Defining [X.sub.iL] as the maximum amount that the ith individual would vote for and [X.sub.iU] to be the lowest amount that the ith individual would not vote for, [WTP.sub.i] lies somewhere in the switching interval [[X.sub.iL] [X.sub.iU]]. Let F([X.sub.i]; [beta]) denote a statistical distribution function for [WTP.sub.i] with parameter vector [beta]. The probability that an individual would vote against a specific dollar amount, [X.sub.i], is simply F([X.sub.i]; 3). Therefore, the probability that a respondent would vote "yes" at a given dollar amount, [X.sub.i] is 1 - F([X.sub.i]; [beta]). The probability that [WTP.sub.i] falls bet ween the two price thresholds, [X.sub.iL] and [X.sub.iU] is F([X.sub.iU]; [beta]) - F([X.sub.iL]; [beta]), resulting in the following log-likelihood function:

lnL = [summation over (n/i=1) ln[F([X.sub.iU]; [beta]) - F([X.sub.iL]; [beta])].

When the respondent says "yes" to every amount, [X.sub.iU] 4. Likewise, when the respondent says "no" to every amount, [X.sub.iL] = -4. It should be apparent that the previous equation represents the log-likelihood function for discrete choice models in general, including the DC model (Welsh and Poe 1998). This likelihood function also parallels that used for analysis of interval data from payment cards (Cameron and Huppert 1989).

Within this framework, WTP functions can he estimated based on any of the voting certainty levels. For example, a "definitely yes" model corresponds to modeling the lower end of the switching interval at the highest amount the individual chose the "definitely yes" response category and the higher end of the switching interval at the next dollar threshold.

3. Description of Data

Data for this paper are taken from a field validity study that collected actual and hypothetical participation commitments to a green electricity program that would fund investments in renewable energy. In 1995-1996, the Niagara Mohawk Power Corporation (NMPC), a public utility in New York State, launched Green Choice[TM], the largest program in the country for the green pricing of electricity (Holt 1997). NMPC's 1.4 million households were offered the opportunity to fund a green electricity program that would invest in renewable energy projects (e.g., landfill gas reclamation, wind power) as substitutes for traditional energy sources and a tree planting program. Such green pricing programs have generated substantial interest as utilities come under increasing pressure to provide alternative sources of electricity for customers who prefer environmentally friendly energy sources (Wiser, Bolinger, and Holt 2000).

Building on the mechanism design recommended by Schulze (1994), NMPC's Green Choice provision mechanism incorporated three key features: a provision point, a money-back guarantee, and extended benefits if excess funds are collected. NMPC customers had the option of signing up for the program at a fixed cost of $6 per month, paid through a surcharge on their electricity bill. If at least $864,000 (the provision point) is collected in the first year, the program is implemented. NMPC would then plant 50,000 trees and fund a landfill gas project that could replace fossil fuel-generated electricity for 1,200 homes. However, if participation were less than $864,000, NMPC would cancel the program and refund all the money that was collected. Any funds collected in excess of the provision point would be applied toward increasing the scope of the program by planting additional trees and hence would extend benefits. The characteristics of the program itself were based on prior market research for NMPC (Wood et al. 1994) . The improved demand revelation characteristics of the program's funding mechanism relative to the standard voluntary contributions mechanisms used in prior field validity research (e.g., Champ et al. 1997) are further discussed in an experimental context in Rondeau, Schulze, and Poe (1999) and Rose et al. (2002). The Rose et al. paper provides additional information on the actual NMPC program and participation levels. In addition, Marks and Croson (1998) provide a detailed discussion of alternative rebate rules for excess contributions and demonstrate empirically that the extended benefits approach used in this research leads to higher contribution rates than no rebate and proportional rebate alternatives.

In the summer of 1996, a telephone survey was conducted using a random sample of households with listed telephone numbers from the NMPC service territory within Erie County. Participants in the phone survey were offered the opportunity to actually sign up for the program at $6 per month, with the charge to appear on their monthly bill. This sign-up-now/pay-later approach follows standard green pricing methods (Holt 1997). Furthermore, the phone solicitation corresponded with the "keep it simple" approach adopted by NMPC, which allowed either phone or mail sign-ups. Because of restrictions by the New York public utilities commission, only a single actual sign-up price of $6 per month was allowed.

In the fall of 1996, a split-sample mail survey was conducted using the same sample population and involved separate DC and MBDC questionnaires in which respondents were asked, hypothetically, whether they would participate in the Green Choice program. Various dollar values were employed, using established bid design methods. In the DC questionnaire, individuals were asked whether they "would sign up for the program if it cost you $___ per month," where the dollar amount was randomly assigned across respondents to be 50Cents, $1, $2, $4, $6, $9, or $12. If they answered "yes," they were asked the follow-up certainty question described previously. MBDC respondents were asked if they "would join the Green Choice program if it would cost you these amounts each month": 10Cents, 50Cents, $1, $1.50, $2, $3, $4, $6, $9, $12, $20, $45, or $95. At each amount, respondents were asked to make a "definitely no," "probably no, not sure," "probably yes," or "definitely yes" response choice. Copies of the questionnaires are available from the authors. Appendix A provides copies of the survey questions. Appendix B provides the distributions of responses to the actual choice, DC, and MBDC questions.

Implementation of the survey instruments followed the Dillman Total Design Method (Dillman 1978). The survey was pretested by administering successive draft versions by phone until respondents clearly understood the instrument. Established multiple contact survey techniques, including a $2 incentive, were used in all versions with Cornell University as the primary correspondent. A private survey research firm, Hagler Bailly, Inc., administered all versions. After adjusting for "list errors" (undeliverables, not NMPC customers, moved out of area, and deceased), adjusted response rates for the hypothetical mail surveys were 66% for the MBDC version and 67% for the DC with follow-up certainty question. The adjusted response rate for the telephone survey was just over 70%. These response rates approximate the 70% response rate guideline established by the NOAA panel report (Arrow et al. 1993).

In each survey version, respondents were first screened to establish that they were NMPC customers and to determine their previous knowledge of the Green Choice program. A description of this program followed, with questions to aid the respondents' understanding. The program description followed the NMPC Green Choice brochure as closely as possible and emphasized various components of the good (trees and renewable energy) and the provision point mechanism. The description was followed by either an actual choice or a CV question, and the survey concluded with demographic questions.

As shown in Table 1, contingency table analyses indicates that the observable demographic characteristics of survey respondents (age, gender, income, completion of a college degree, and whether the respondent has contributed to any environmental group in the last two years) are not statistically different across the three sample groups at the 5% significance level. (1) Hence, any procedural variance observed can be attributed to how respondents answer different questions and not to sample selection.

4. Empirical Results

Logistic response functions for the FCQ responses are reported in the top portion of Table 2. Corresponding estimates for MBDC responses are presented in Table 3. Estimates of participation at $6 (the cost of actually signing up) and mean WTP estimates for nonnegative values, following Hanemann (1984, 1989), are reported for each model. Ninety-five percent confidence intervals for the participation and mean WTP estimates from the parametric models are estimated using the Krinsky and Robb (1986) procedure with 10,000 random draws. In the bottom portion of Tables 2 and 3, nonparametric estimates of participation at $6 and mean WTP are calculated using Kristrom's (1990) approach. (2) Confidence intervals for the nonparametric estimates are obtained by creating 10,000 normally distributed random draws using the mean and variance of the estimates. In the following subsections, we examine different hypotheses about criterion validity, replicability, and convergent validity, using the participation rates at $6, mean WTP estimates, and WTP distributions as the respective measures of interest.

Criterion Validity: A Comparison with Actual Participation Decisions

In the telephone survey actual sign-ups were collected, resulting in a participation rate at $6 of 20.4%. This value serves as a criterion for assessing the predictive power of each method. It should be noted that the actual participation rates used here greatly exceed expected sign-ups for green electricity programs in the field because our sample is, by necessity, completely aware of the existence of the program. Such 100% awareness greatly differs from the limited consumer awareness typically associated with green pricing programs. Also, a potential concern is the possible differences between phone and mall elicitation methods. Phone contingent valuation responses were collected as part of a larger research effort (see Ethier et al. 2000), and comparability between hypothetical phone and mail responses suggests that the differences in elicitation formats are not a problem.

Using the 20.4% actual sign-up rate as the reference criterion, we see that the MBDC "probably yes" (parametric: 19.8%; nonparametric: 17.8%) and DC Cert [greater than or equal to] 7 (parametric: 22.0%; nonparametric: 19.3%) models are the closest predictors of actual sign-ups. To assess significance, a distribution of actual participation was simulated using the binomial distribution, and the convolutions method (Poe, Severance-Lossin, and Welsh 1994) was employed to compare distributions. These methods indicate that the Pr(yes) at $6 for the MBDC "probably yes" model are not significantly different from the actual participation rate (parametric [[p.sub.p]]: [p.sub.p] = 0.903; nonparametric [[]]: [] = 0.539). The DC Cert [greater than or equal to] 6 ([p.sub.p] = 0.310; [] = 0.805), DC Cert [greater than or equal to] 7 ([p.sub.p] = 0.682; [] = 0.789), and DC Cert [greater than or equal to] 8 ([p.sub.p] = 0.532; [] 0.306) models were also not significantly different from actual participation rates, although the DC Cert [greater than or equal to] 7 provides the best predictor under both the parametric and nonparametric specifications. All other comparisons of calibrated hypothetical responses with actual responses are significantly different at the 5% level.

Replication of Welsh and Poe

In their recent empirical investigation, Welsh and Poe found that DC response patterns corresponded closely with the "not sure" MBDC model, suggesting that individuals who are unsure about their response to a dollar amount would tend to vote "yes" to a DC question. A potential concern about the Welsh and Poe study is that it was conducted in a classroom setting. Here we examine if these results are replicated in the field.

In contrast to the Welsh and Poe study, DC values do not correspond with the MBDC "not sure" model but instead lie between the point estimates of the "probably yes" and the "not sure" models. Using the convolutions approach, the null hypothesis of identical mean WTP between the "not sure" model and the DC model is rejected for both the parametric and nonparametric specifications ([p.sub.p] < 0.001; [] = 0.000). Equality of mean WTP between the DC and the "probably yes" ([p.sub.p] < 0.001; [] < 0.001) and "definitely yes" ([p.sub.p] = 0.000; [] = 0.000) models is also rejected. The Pr(yes) at $6 from the DC models are also significantly different from the "definitely yes" ([p.sub.p] = 0.000; [] = 0.000), "probably yes" ([p.sub.p] < 0.001; [] 0.001), and "not sure" ([p.sub.p] < 0.001; [] = 0.403) model estimates except when the nonparametric DC and "not sure" model values are compared. The correspondence between the nonparametric DC and "not sure" model is coincid ental, however, as the empirical cumulative density functions are really very different. Using the Smirnov test (Conover 1980), we reject the null hypothesis of identical distributions (D = 0.151, p < 0.01). Thus, although our specific results do not concur with those of Welsh and Poe, the critical message from their article remains: DC response patterns correspond with values that have a relatively low level of voting certainty.

Convergent Validity: Comparing Certainty Corrections across Methods

We now compare certainty corrections across the FCQ and MBDC methods. For example, does the "definitely yes" response to the MBDC question format correspond with high levels of certainty in the FCQ and so on? Consistent with expectations, the Pr(yes) at $6 declines as the certainty level increases, and the mean WTP and Pr(yes) at $6 is inversely related to certainty levels. A comparison of these models indicates that the MBDC "definitely yes" model corresponds closely with the DC Cert [greater than or equal to] 9 model (mean WTP: [p.sub.p] = 0.820, [] = 0.532; Pr(yes) at $6: [p.sub.p] = 0.100; [] = 0.595). The mean WTP and Pr(yes) at $6 of the MBDC "probably yes" parametric and nonparametric models most closely corresponds with the DC Cert [greater than or equal to] 7 models (mean WTP: [p.sub.p] = 0.852, [] = 0.624; Pr(yes) at $6: [p.sub.p] 0.468; [] = 0.699) and are also not statistically different at the 5% level from the DC Cert [greater than or equal to] 6 (mean WTP: [p.sub .p] = 0.283, [] = 0.562; Pr(yes) at $6: [p.sub.p] = 0.145; [] = 0.330) and DC Cert [greater than or equal to] 8 models (mean WTP: [p.sub.p] = 0.176, [] = 0.010; Pr(yes) at $6: [p.sub.p] = 0.527; [] = 0.650) except when mean WTP is compared between the "probably yes" and Cert [greater than or equal to] 8 nonparametric models. As indicated earlier, the "not sure" model already exceeds the standard DC analysis and thus is not comparable to any of the corrected measures. In general, the models that are good predictors of the actual participation rate--the "probably yes" model and the DC Cert [greater than or equal to] 6, DC Cert [greater than or equal to] 7, and DC Cert [greater than or equal to] 8 models--seem to correspond closely with each other.

Even though it appears that there is a close correspondence between MBDC and DC models in terms of their certainty-corrected responses, this similarity is merely coincidental and dependent on the values (i.e., the nonnegative mean WTP and Pr(Yes) at $6) examined. Using the Smirnov test, the equality of the "definitely yes" and DC Cert [greater than or equal to] 9 nonparametric distributions is strongly rejected ([] 0.189, p < 0.01) even though we found equality between the nonnegative mean WTP and Pr(yes) at $6. Using a Kolmogorov-Smirnov test (Conover 1980), the equality of the parametric distributions for these same models is also rejected ([D.sub.p] = 0.214, p < 0.01). Equality of distributions is likewise strongly rejected when comparing the "probably yes" model with the DC Cert [greater than or equal to] 6 ([[D.sub.p] = 0.l67, p < 0.01; [] = 0.151, p < 0.01), the DC Cert [greater than or equal to] 7 ([D.sub.p] = 0.15l, p < 0.01; [] 0.l93, p < 0.01), and DC Cert [greater than or eq ual to] 8 ([D.sub.p] = 0.271, p < 0.01; [] = 0.28 1, p < 0.0l) models.

To further demonstrate the difference in underlying WTP distributions, the top portion of Figure 1 shows the positive domain of the estimated parametric distributions for the different DC certainty levels. Figure 2 shows the estimated distributions for the different multiple-bounded models. As the certainty levels increase, the DC response functions shift downward, and the Pr(yes) at $0 and other values shift downward dramatically. In general, as the DC certainty level increases, the "constant" of the model decreases, while the "slope" is largely unchanged. In contrast, it appears that as the certainty level increases within the multiple-bounded format, the response function shifts inward and becomes much steeper. The downward effect on the Pr(yes) at $0 is not as notable, with even the "definitely yes" model crossing the axis above the 50th percentile. In general, as the MBDC certainty level increases, the change in the "constant" is ambiguous, while the "slope" consistently increases. Thus, although both me thods seek to measure a certainty-corrected value, it is clear that the response functions they elicit are fundamentally different, as the DC correction affects primarily the "constant," and the MBDC correction impacts the "slope."

The equality of certainty corrections with each other and with actual participation at $6 appears to be merely coincidental. This point is demonstrated in Figure 3, which overlays the multiple-bounded "probably yes" model with the DC Cert [greater than or equal to]7 model. As depicted, the percentage of "yes" responses is much lower for the DC Cert [greater than or equal to] 7 model at low bid amounts than the multiple-bounded "probably yes" model. The reverse is true for high dollar amounts. The two functions cross at around $5.23, and the difference between the two distributions is small only for very limited range of bids, including $6.

5. Concluding Remarks

Two methods for calibrating discrete choice contingent valuation responses--the dichotomous choice with follow-up certainty question method of Champ et al. (1997) and the multiple-bounded method of Welsh and Poe (1998)--are evaluated using data from a field validity comparison of hypothetical and actual participation decisions in a green electricity pricing program. Treating MBDC "probably yes" responses and DC responses with an associated certainty level of 6 and higher, 7 and higher, or 8 and higher to be "yes" responses leads to hypothetical program participation rates that are not statistically different from actual participation rates. As such, our findings coincide with those of other researchers who find that hypothetical responses tend to overstate WTP and that appropriate certainty corrections correspond with a moderate to high rate of certainty.

Contrary to Welsh and Poe, our MBDC "not sure" model does not coincide with the DC model. However, we do find that DC responses reflect low levels of certainty if we take the uncertainty expressed in MBDC responses as truth. Hence, while the specific statistical correspondence observed in Welsh and Poe does not apply here, the basic result that DC responses correspond with relatively low levels of payment certainty is replicated.

Further exploration of the various discrete choice models reveals that even though some MBDC models and DC models with certainty corrections are not statistically different in terms of their program participation rate predictions and mean WTP estimates, the underlying WTP distributions are significantly different. This suggests that the underlying behavioral models are fundamentally distinct and that the two correction methods do not coincide. Because regulatory restrictions prevented the collection of actual program sign-ups at multiple prices, we are unable to examine how actual contributions vary across prices in this research. Based on our results, however, it appears that such comparisons offer a critical area of future research.

Of obvious interest is which of these methods should be used in future studies. On the basis of this single study, it would be premature for us to provide a definitive answer. We instead conclude by highlighting some theoretical and empirical trade-offs that may be important when considering alternative correction methods.

Given that the DC with follow-up certainty question and the MBDC methods suggest different implied WTP distributions, one way of comparing these methods is to look at how well the implied distributions fit with a priori theoretical expectations. In this case, it seems reasonable to expect that no one would have a negative WTP for the Green Choice program and that the predicted probability of a "yes" response at $0 would be quite high. The DC model with Certainty [greater than or equal to] 7 produces an estimated probability(yes) of about 0.5 at a price of $0, while the MBDC "probably yes" model produces a probability(yes) of about 0.7. On this basis, we might judge the revealed demand of the MBDC "probably yes" model as having greater consistency with theoretical expectations. At the same time, the DC format, which offers a single "yes/no" decision opportunity, is most consistent with the theoretical concept of incentive compatibility (Carson, Groves, and Machina 1999). As such, there is no clear preference b etween methods on theoretical grounds.

In a similar vein, practical and empirical considerations involve trade-offs across methods. The DC format is less demanding on respondents, who are familiar with take-it-or-leave-it decision making in standard market transactions. However, the MBDC format allows the researcher to observe information on several points rather than a single point of the respondent's WTP distribution. This increases the statistical efficiency of WTP estimates. The opportunity to elicit each respondent's WTP for several dollar values also decreases the importance of optimal bid design, which is a widely discussed issue in DC methodology (for a definitive review of these issues, see Hanemann and Kanninen 1999). Whether the dollar values presented to respondents in a MBDC format influence their answers to the individual amounts remains an open empirical question. Whereas Roach, Boyle, and Welsh (2002) and Vossler et al. (2002) provide evidence against such bid design effects by comparing groups of survey respondents receiving diffe rent dollar values, Alberini, Boyle, and Welsh (in press) find that the response distributions and WTP estimates are quite sensitive to whether dollar values are presented in ascending versus descending order.

Appendix A

Actual Sample (Phone):

You may need a moment to consider the next couple of questions. Given your household income and expenses, I'd like you to think about whether or not you would be interested in the Green Choice program. If you decide to sign up, we will send your name to Niagara Mohawk and get you enrolled in the program. All your other answers to this survey will remain confidential.

Does your household want to sign up at a cost of $6 per month?

1. Yes

2. No.

Hypothetical Dichotomous Choice with Follow-Up Choice Question (mail Sample $6)

Given your household's income and other expenses, we would like you to think about whether or not you would be interested in joining the Green Choice program.

10. Would your household sign up for the program if it cost you $6 per month? (Please circle ONE response)

1 Yes

2 No ---------> Skip to Question 12 on the next page.

11. So you think that you would sign up. We would like to know how sure you are of that. On a scale from '1' to '10', where '1' is 'Very Uncertain' and '10' 'Very Certain', how certain are you that you would sign up and pay the extra $6 a month if the program were actually offered? (Please circle ONE response)
 Very Very
Uncertain Certain

 1 2 3 4 5 6 7 8 9 10

Hypothetical Multiple Bounded Discrete Choice Question (mail)

Given your household's income and other expenses, we would like you to think about whether or not you would be interested in joining the Green Choice program.

10. Would you join the Green Choice program if it would cost you these amounts each month?
(Please circle ONE letter for EACH dollar amount to show if you would

Cost to You Definitely Probably Not Probably Definitely
per Month No No Sure Yes Yes

10cents A B C D E
50cents A B C D B
$1 A B C D E
$1.50 A B C D E
$2 A B C D E
$3 A B C D E
$4 A B C D E
$6 A B C D E
$9 A B C D E
$12 A B C D E
$20 A B C D E
$45 A B C D E
$95 A B C D E

Appendix B

Distribution of Survey Responses

Actual Phone Responses

 Price % Yes
 $6 20.42 (29/142)

Discrete Choice Discrete Choice Responses
 Responses with Certainty

 % Yes
Price % Yes With Cert [greater than
 or equal to] 5

$0.50 65.04 (80/123) 62.60 (77/123)
$1 62.04 (67/108) 59.26 (64/108)
$2 47.90 (57/119) 45.38 (54/119)
$4 21.93 (25/114) 19.30 (22/114)
$6 38.53 (42/109) 33.94 (37/109)
$9 20.39 (21/103) 18.45 (19/103)
$12 13.68 (16/117) 11.97 (14/117)

 % Yes
 20.42 (29/142)

 Discrete Choice Responses with Certainty Corrections

 % Yes % Yes
Price With Cert [greater than or equal With Cert [greater than
 to] 6 or equal to] 7

$0.50 60.16 (74/123) 54.47 (67/123)
$1 50.00 (54/108) 47.22 (51/108)
$2 41.18 (49/119) 37.82 (45/119)
$4 17.54 (20/114) 16.67 (19/114)
$6 25.69 (28/109) 22.02 (24/109)
$9 16.50 (17/103) 13.59 (14/103)
$12 11.11 (13/117) 11.11 (13/117)

 % Yes
 20.42 (29/142)

 Discrete Choice Responses with Certainty

 % Yes % Yes
Price With Cert [greater than With Cert [greater than
 or equal to] 8 or equal to] 9

$0.50 48.78 (60/123) 38.21 (47/123)
$1 37.04 (40/108) 27.78 (30/108)
$2 33.61 (40/119) 23.53 (28/119)
$4 13.16 (15/114) 10.53 (12/114)
$6 19.27 (21/109) 10.09 (11/109)
$9 9.71 (10/103) 5.83 (6/103)
$12 9.40 (11/117) 4.27 (5/117)

 % Yes
 20.42 (29/142)

 Discrete Choice Responses
 with Certainty

 % Yes
Price With Cert [greater than
 or equal to] 10

$0.50 28.46 (35/123)
$1 21.30 (23/108)
$2 16.81 (20/119)
$4 6.14 (7/114)
$6 7.34 (8/109)
$9 2.91 (3/103)
$12 2.56 (3/117)

Multiple-Bounded Discrete Choice Responses

Price % Not Sure % Probably Yes % Definitely Yes

$0.10 80.66 (196/243) 76.54 (186/243) 60.91 (148/243)
$0.50 80.18 (182/227) 74.45 (169/227) 54.19 (123/227)
$1 74.34 (168/226) 65.93 (149/226) 47.35 (107/226)
$1.50 66.22 (147/222) 52.70 (117/222) 35.14 (78/222)
$2 59.91 (133/222) 45.95 (102/222) 28.83 (64/222)
$3 48.64 (107/220) 33.64 (74/220) 18.18 (40/220)
$4 44.14 (98/222) 27.03 (60/222) 15.77 (35/222)
$6 33.79 (74/219) 17.81 (39/219) 8.22 (18/219)
$9 21.56 (47/218) 11.93 (26/218) 4.59 (10/218)
$12 14.22 (31/218) 5.96 (13/218) 2.29 (5/218)
$20 7.37 (16/217) 1.84 (4/217) 0.92 (2/217)
$45 3.67 (8/218) 0.00 (0/218) 0.00 (0/218)
$95 2.29 (5/218) 0.00 (0/218) 0.00 (0/218)



Table 1

Comparisons of Respondent Characteristics across Samples

 Chi-Squared Actual Mean MBDC Mean
Variable (d.f.) N (n) (n)

Age 31.326 (a) 1177 55.66 51.58
 (22) (135) (252)
Gender 4.067 1209 44.37% male 52.31% male
 (2) (142) (260)
Income 14.980 (b) 1107 $41,849 $44,071
 (10) (119) (240)
College degree 3.439 1190 45.00% 35.55%
 (2) (140) (256)
Give to environmental 1.213 1203 19.15% 19.62%
 groups (2) (141) (260)

 DC Mean
Variable (n)

Age 52.52
Gender 53.53% male
Income $41,188
College degree 38.41%
Give to environmental 22.19%
 groups (802)

(a) Age is a continuous variable but is converted to the following
categories: [less than or equal to]30, 31-35, 36-40, 41-45 ... 76- 80,
and above 80. The upper and lower age categories are wider so that there
are enough phone survey responses ([greater than or equal to]5) in them.

(b) In the survey, income categories are as follows: under $15,000,
$15,000 to $30,000, $30,000 to $50,000, $50,000 to $75,000, $75,000 to
$100,000, $100,000 to $150,000, $150,000 to $250,000, and $250,000 or
over. The highest three categories are pooled for the chi-squared test,
as there are very few phone survey responses in them.

* and ** correspond with 5% and 1% levels of significance, respectively.
In this case, none of the chi-squared values are significant at these

Table 2

Dichotomous Choice with Certainty Corrections

 Cert [greater than
Model Uncorrected or equal to] 5

Parametric estimation (logit)
 "Constant" 0.455 0.356
 ([alpha]) (0.117) ** (0.117) **
 "Slope" -0.207 -0.215
 ([beta]) (0.022) ** (0.023) **
Wald statistic 85 ** 85 **
 N 793 793
 Pr(yes) at $6 0.313 0.282
 [95% CI] [0.276, 0.352] [0.246, 0.321]
 Mean WTP 4.57 4.13
 [95% CI] [4.01, 5.35] [3.61, 4.84]
Nonparametric estimation (Kristrom)
 Pr(yes) at $6 0.300 0.265
 [95% CI] [0.240, 0.359] [0.207, 0.321]
 Mean WTP 4.47 4.09
 [95% CI] [3.99, 4.95] [3.63, 4.54]

 Cert [greater than
Model or equal to] 6

Parametric estimation (logit)
 "Constant" 0.147
 ([alpha]) (0.118)
 "Slope" -0.213
 ([beta]) (0.024) **
Wald statistic 76 **
 N 793
 Pr(yes) at $6 0.244
 [95% CI] [0.209, 0.281]
 Mean WTP 3.61
 [95% CI] [3.13. 4.28]
Nonparametric estimation (Kristrom)
 Pr(yes) at $6 0.215
 [95% CI] [0.161, 0.268]
 Mean WTP 3.62
 [95% CI] [3.17, 4.05]

 Cert [greater than
Model or equal to] 7

Parametric estimation (logit)
 "Constant" -0.014
 ([alpha]) (0.120)
 "Slope" -0.209
 ([beta]) (0.025) **
Wald statistic 68 **
 N 793
 Pr(yes) at $6 0.220
 [95% CI] [0.187, 0.256]
 Mean WTP 3.28
 [95% CI] [2.82, 3.94]
Nonparametric estimation (Kristrom)
 Pr(yes) at $6 0.193
 [95% CI] [0.141, 0.243]
 Mean WTP 3.33
 [95% CI] [2.90, 3.75]

 Cert [greater than
Model or equal to] 8

Parametric estimation (logit)
 "Constant" -0.272
 ([alpha]) (0.124) *
 "Slope" -0.208
 ([beta]) (0.027) **
Wald statistic 58 **
 N 793
 Pr(yes) at $6 0.180
 [95% CI] [0.149, 0.215]
 Mean WTP 2.73
 [95% CI] [2.31, 3.34]
Nonparametric estimation (Kristrom)
 Pr(yes) at $6 0.161
 [95% CI] [0.113, 0.208]
 Mean WTP 2.81
 [95% CI] [2.41, 3.20]

 Cert [greater than
Model or equal to] 9 Cert = 10

Parametric estimation (logit)
 "Constant" -0.621 -0.989
 ([alpha]) (0.137) ** (0.154) **
 "Slope" -0.253 -0.277
 ([beta]) (0.035) ** (0.044) **
Wald statistic 51 ** 39 **
 N 793 793
 Pr(yes) at $6 0.106 0.066
 [95% CI] [0.081, 0.137] [0.046, 0.093]
 Mean WTP 1.70 1.14
 [95% CI] [1.42, 2.13] [0.92, 1.478]
Nonparametric estimation (Kristrom)
 Pr(yes) at $6 0.101 0.067
 [95% CI] [0.045, 0.156] [0.035, 0.099]
 Mean WTP 1.87 1.34
 [95% CI] [1.54, 2.20] [1.09, 1.57]

Standard errors are in parentheses.

* and ** correspond to 5% and 1% significance levels, respectively.

Table 3

Multiple-Bounded Discrete Choice Models

 Definitely Probably
Model Yes Yes

Parametric estimation (logit)
 "Constant" 0.258 0.866
 ([alpha]) (0.117) * (0.120) **
 "Slope" -0.471 -0.377
 ([beta]) (0.036) ** (0.026) **
 Wald statistic 166 ** 213 **
 N 260 260
 Pr(yes) at $6 0.071 0.198
 [95% CI] [0.049, 0.104] [0.156, 0.248]
 Mean WTP 1.76 3.23
 [95% CI] [1.49, 2.11] [2.80, 3.73]

Nonparametric estimation (Kristrom)
 Pr(yes) at $6 0.082 0.178
 [95% CI] [0.046, 0.118] [0.128, 0.227]
 Mean WTP 2.00 3.46
 [95% CI] [1.79, 2.20] [3.17, 3.74]

Model Sure

Parametric estimation (logit)
 "Constant" 0.745
 ([alpha]) (0.113) **
 "Slope" -0.159
 ([beta]) (0.011) **
 Wald statistic 202 **
 N 260
 Pr(yes) at $6 0.448
 [95% CI] [0.398, 0.500]
 Mean WTP 7.14
 [95% CI] [6.16, 8.31]

Nonparametric estimation (Kristrom)
 Pr(yes) at $6 0.338
 [95% CI] [0.275, 0.399]
 Mean WTP 8.35
 [95% CI] [7.10, 9.58]

Standard errors are in parentheses.

* and ** correspond to 5% and 1% significance levels, respectively.

Received November 2001; accepted August 2002.

(1.) Age is statistically different across samples at the 10% level. However, when we compare just the MBDC and DC samples, no characteristic is different at the 10% level. Most of the focus in this paper is on analyzing survey responses from these two groups.

(2.) We estimate the variance of WTP (and, subsequently, confidence intervals) based on Haab and McConnell's (1997, p. 259) derivation of the variance for the Tumbull estimator. That is, we treat the proportion of "yes" responses to each dollar amount as random variables. This is contrary to the approach of Boman, Bostedt, and Kristrom (1999), which treats the dollar amounts themselves as random variables. Formally, using Haab and McConnell's notation, the variance is given by

[summation over (M+1/j=1)][[0.5*[c.sub.j-1]+0.5*[c.sub.j]].sup.2](V([F.sub.j]+[F.sub. j-1])]-2[summation over M/j=1)][0.5*[c.sub.j-1]+0.5*[c.sub.j]*[0.5*[c.sub.j]+0.5*[c.sub.j+1]] *V([F.sub.j])

where [c.sub.j] denotes the dollar amount, [F.sub.j] denotes the cumulative density function at dollar amount j, and V(*) denotes a variance.


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Christian A. Vossler *, Robert G. Ethier +, Gregory L. Poe ++ and Michael P. Welsh (ss)

* Department of Applied Economics and Management, Cornell University, 157 Warren Hall, Ithaca, NY 14853, USA; E-mail

+ ISO-New England, Holyoke, MA 01040, USA; E-mail

++ Department of Applied Economics and Management, Cornell University, 454 Warren Hall, Ithaca, NY 14853, USA; E-mail; corresponding author.

(ss) Christensen Associates, 4610 University Avenue, Madison, WI 53705, USA; E-mail

We are grateful to William Schulze, Jeremy Clark, Daniel Rondeau, Steve Rose, and Eleanor Smith for their input into various components of this researeh. We also wish to thank Theresa Flaim, Janet Dougherty, Mike Kelleher, Pam Ingersol, and Mana Ucchino at Niagara Mohawk Power Corporation for facilitating this research and Pam Rathbun and colleagues at Hagler Bailly, Inc., Madison, Wisconsin, for their survey expertise. Any errors, however, remain our responsibility. Funding for this project was provided by NSF/EPA Grant R 824688 and USDA Regional Project W-133, Cornell University.
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