Report of the NOAA Panel on Contingent Valuation

January 11, 1993

Kenneth Arrow

Robert Solow

Paul R. Portney

Edward E. Leamer

Roy Radner

Howard Schuman
Report of the NOAA Panel on Contingent Valuation

Date: May 9, 2001

I. INTRODUCTION

Under the Oil Pollution Act of 1990, the President--acting

through the Under Secretary of Commerce for Oceans and

Atmosphere--is required to issue regulations establishing

procedures for assessing damages to or destruction of natural

resources resulting from a discharge of oil covered by the Act.

These procedures are to ensure the recovery of restoration costs

as well as the diminution in value of the affected resources and

any reasonable costs of conducting the damage assessment.

At least some of the values that might be diminished by such

a discharge are relatively straightforward to measure through

information revealed in market transactions. For instance, if

the discharge kills fish and thereby reduces the incomes of

commercial fishermen, their losses can reasonably be calculated

by the reduced catch multiplied by the market price(s) of the

fish (less, of course, any costs they would have incurred). 

Similarly, if the discharge of oil discourages tourist travel to

an area, the lost incomes of those owning and/or operating

motels, cottages, or other facilities can be reasonably

represented by the difference in revenues between the affected

period and a "normal" season. Even the losses to recreational

fishermen, boaters, swimmers, hikers, and others who make active

use of the areas affected by the discharge can be included in the

estimate of diminished value, although these losses will
2

generally be somewhat more difficult to value than the more

obvious out-of-pocket losses.

The losses described above have come to be known as lost

"use values" because they are experienced by those who, in a

variety of different ways, make active use of the resources

adversely affected by the discharge. But for at least the last

twenty-five years, economists have recognized the possibility

that individuals who make no active use of a particular beach,

river, bay, or other such natural resource might, nevertheless,

derive satisfaction from its mere existence, even if they never

intend to make active use of it.

This concept has come to be known as "existence value" and

it is the major element of what are now referred to as "non-use"

or "passive-use" values (the latter term is employed in the

balance of this report). In regulations promulgated by the

Department of the Interior in 1986 under the Comprehensive

Environmental Response, Compensation, and Liability Act --

regulations that also pertained to natural resource damage

assessments -- passive-use values were included among the losses

for which trustees could recover. The inclusion of passive-use

values was recently upheld by the D. C. Court of Appeals (State

of Ohio v. Department of the Interior, 880 F.2d 432 (D.C. Cir.

1989)), as long as they could be reliably measured.

This begs an interesting and important question, however. 

If passive-use values are to be included among the compensable

losses for which trustees can make recovery under the Oil
3

Pollution Act, how will they be estimated? Unlike losses to

commercial fishermen or recreational property owners, there are

no direct market transactions that can be observed to provide

information on which estimates can be based. Unlike losses to

boaters, swimmers, recreational fishermen and others, there exist

no indirect methods through which market data can provide at

least some clues as to lost values. In other words, there appear

to be neither obvious nor even subtle behavioral trails that can

provide information about lost passive-use values.

Some experts believe that there exists an approach that can

provide useful information about the economic significance of the

lost passive-use values individuals may suffer when oil

discharges damage natural resources. Known as the contingent

valuation (or CV) technique, this approach is based on the direct

elicitation of these values from individuals through the use of

carefully designed and administered sample surveys. Its appeal

lies in its potential to inform damage assessment in an area

(lost passive-use values) where there appear to be no behavioral

trails to be followed.

Typically, CV studies provide respondents with information

about a hypothetical government program that would reduce the

likelihood of a future adverse environmental event such as an oil

spill, chemical accident, or the like. Respondents are usually

given some specific information about the exact nature of the

damages that the program in question would prevent. And they are

also confronted in the study with a question or questions that
4

provide information about the economic sacrifice they would have

to make to support the environmental program. This may take the

form of an open-ended question asking what is the maximum amount

they would be willing to pay for the program in question; it may

involve a series of questions confronting them with different

prices for the program depending on their previous answers; or it

may take the form of a hypothetical referendum (like a school

bond issue) in which respondents are told how much each would

have to pay if the measure passed and are then asked to cast a

simple "yes" or "no" vote. (The conceptually correct measure of

lost passive-use value for environmental damage that has already

occurred is the minimum amount of compensation that each affected

individual would be willing to accept. Nevertheless, because of

concern that respondents would give unrealistically high answers

to such questions, virtually all previous CV studies have

described scenarios in which respondents are asked to pay to

prevent future occurrences of similar accidents. This is the

conservative choice because willingness to accept compensation

should exceed willingness to pay, if only trivially; we say more

about other biases below.)

The CV technique has been used for twenty years or so to

estimate passive-use values. In the last five years, however,

there has been a dramatic increase in the number of academic

papers and presentations related to the CV technique. This is

due in part to the availability of comprehensive reference texts

on the subject (Mitchell and Carson (1989), for instance), and to
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the growing interest both nationally and internationally in

environmental problems and policies. But it is also attributable

to the growing use of the CV technique in estimating lost

passive-use values in litigation arising from state and federal

statutes designed to protect natural resources. Since Ohio v.

Department of the Interior admitted the concept of passive-use

values in damage assessments, this can only give added impetus to

the use of CV in such litigation.

The CV technique is the subject of great controversy. Its

detractors argue that respondents give answers that are

inconsistent with the tenets of rational choice, that these

respondents do not understand what it is they are being asked to

value (and, thus, that stated values reflect more than that which

they are being asked to value), that respondents fail to take CV

questions seriously because the results of the surveys are not

binding, and raise other objections as well. Proponents of the

CV technique acknowledge that its early (and even some current)

applications suffered from many of the problems critics have

noted, but believe that more recent and comprehensive studies

have already or soon will be able to deal with these objections.

This (sometimes acrimonious) debate has put the National

Oceanic and Atmospheric Administration (NOAA) in a very difficult

spot. NOAA must decide in promulgating the regulations under the

Oil Pollution Act whether the CV technique is capable of

providing reliable information about lost existence or other

passive-use values. Toward this end, NOAA appointed the
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Contingent Valuation Panel to consider this question and make

recommendations to it.

This report is the product of the Panel's deliberations

and is organized in the following way. Following this

introduction, the drawbacks to the CV technique are discussed in

Section II. Section III discusses several key issues concerning

the design of CV surveys, including use of the referendum format

to elicit individual values, ways of addressing the so-called

"embedding" problem, and the evaluation of damages that last for

some period but not forever. Section IV presents guidelines to

which the Panel believes any CV study should adhere if the study

is to produce information useful in natural resource damage

assessment. (These are elaborated upon in an Appendix.) In

Section V a research agenda is described; it is the Panel's

belief that future applications of the CV technique may be less

time-consuming and contentious if the research described in the

agenda is carried out. Section VI presents the Panel's

conclusions.

II. CRITICISMS OF THE CONTINGENT VALUATION METHOD

The contingent valuation method has been criticized for many

reasons and the Panel believes that a number of these criticisms

are particularly compelling. Before identifying and discussing

these problems, however, it is worth pointing out that they all

take on added importance in light of the impossibility of

validating externally the results of CV studies. It should be
7

noticed, however, that this same disadvantage must inhere in any

method of assessing damages from deprivation of passive-use. It

is not special to the CV approach although, as suggested in

Section I, there are currently no other methods capable of

providing information on these values.

One way to evade this difficulty, at least partially, is to

construct experiments in which an artificial opportunity is

created to pay for environmental goods. The goods in question

can perfectly well involve passive use. Then the results of a CV

estimate of willingness to pay can be compared with the "real"

results when the opportunity is made available to the same sample

or an analogous sample.

A few such experiments have been attempted. The most

recent, due to Seip and Strand (1992), used CV to estimate

willingness to pay for membership in a Norwegian organization

devoted to environmental affairs, and compared this estimate with

actual responses when a number of the same respondents were

presented with an opportunity actually to contribute. The

finding was that self-reported willingness to pay was

significantly greater than "actual" willingness to pay. A recent

study by Duffield and Patterson (1991) took as the environmental

amenity in question the maintenance of stream flow in two Montana

rivers. The rivers in question provided spawning grounds for two

rare species of fish; passive use was believed to be the main

motivation for respondents. One of two parallel samples was

asked about hypothetical willingness to contribute to the Montana
8

Nature Conservancy which would then maintain stream flow; the

other was offered an opportunity actually to contribute to the

same organization for the same purpose. It was found that

response rates and expressed willingness to contribute were

significantly higher when the contribution was hypothetical than

when "expressed willingness" meant an immediate cash

contribution. On the other hand, the size of contributions,

hypothetical in one case and actual in the other, was not much

different as between those who said they would contribute and

those who did so.

These studies suggest that the CV technique is likely to

overstate "real" willingness to pay. Duffield and Patterson,

however, hold out hope that the differences are small enough and

predictable enough that CV estimates could be discounted for

possible overstatement and then used as a conservative estimate

of willingness to pay. Clearly more such experiments would be

useful.

A less direct test of the "reality" of CV estimates of lost

passive use values is to use the technique to estimate

willingness to pay for ordinary market goods and then to compare

the results with actual purchases. This has been tried by

Dickie, Fisher, and Gerking (1987) using the demand for

strawberries. When the data were re-analyzed by Diamond,

Hausman, Leonard, and Denning (1992), it was found that the CV

approach tended systematically to overestimate quantity demanded

at each price, sometimes by as much as 50 percent. This result
9

has to be qualified in two ways. First, the original CV study

seems to have been fairly casual by the standards now proposed by

practitioners; pre-testing and improvement of the survey

instrument might (perhaps) have narrowed the gap. And second, it

seems to go too far to conclude from systematic over-estimation

that the CV study, even as conducted, provides no information

about the demand for strawberries. Much of the same could be

said about a study submitted to the Panel by Cummings and

Harrison (1992) comparing hypothetical and demonstrated

willingness to pay for small household goods. (See also Bishop

and Heberlein (1979).)

External validation of the CV method remains an important

issue. A critically important contribution could come from

experiments in which state-of-the-art CV studies are employed

in contexts where they can in fact be compared with "real"

behavioral willingness to pay for goods that can actually be

bought and sold.

Of the other problems arising in CV studies, the following

are of most concern to the Panel: (i) the contingent valuation

method can produce results that appear to be inconsistent with

assumptions of rational choice; (ii) responses to CV surveys

sometimes seem implausibly large in view of the many programs for

which individuals might be asked to contribute and the existence

of both public and private goods that might be substitutes for

the resource(s) in question; (iii) relatively few previous

applications of the CV method have reminded respondents
10

forcefully of the budget constraints under which all must

operate; (iv) it is difficult in CV surveys to provide adequate

information to respondents about the policy or program for which

values are being elicited and to be sure they have absorbed and

accepted this information as the basis for their responses; 

(v) in generating aggregate estimates using the CV technique, it

is sometimes difficult determining the "extent of the market;"

and (vi) respondents in CV surveys may actually be expressing

feelings about public spiritedness or the "warm glow" of giving,

rather than actual willingness to pay for the program in

question. We discuss each of these briefly.

Inconsistency with Rational Choice

Some of the empirical results produced by CV studies have

been alleged to be inconsistent with the assumptions of rational

choice. This raises two questions: What requirements are

imposed by rationality? Why are they relevant to the evaluation

of the reliability of the CV method?

Rationality in its weakest form requires certain kinds of

consistency among choices made by individuals. For instance, if

an individual chooses some purchases at a given set of prices and

income, then if some prices fall and there are no other changes,

the goods that the individual would now buy would make him or her

better off. Similarly, we would expect an individual's

preferences over public goods (i.e., bridges, highways, air

quality) to reflect the same kind of consistency.
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Common notions of rationality impose other requirements

which are relevant in different contexts. Usually, though not

always, it is reasonable to suppose that more of something

regarded as good is better so long as an individual is not

satiated. This is in general translated into a willingness to

pay somewhat more for more of a good, as judged by the

individual. Also, if marginal or incremental willingness to pay

for additional amounts does decline with the amount already

available, it is usually not reasonable to assume that it

declines very abruptly.

 This point assumes importance in view of some empirical

evidence from CV studies that willingness to pay does not

increase with the good. In one study, Kahneman (1986) found that

willingness to pay for the cleanup of all lakes in Ontario was

only slightly more than willingness to pay for cleaning up lakes

in just one region. Evidence of this kind has multiplied (see

Kahneman and Knetch (1992), Desvousges, et al. (1992), and

Diamond et al. (1992)). Desvousges' result is very striking; the

average willingness to pay to take measures to prevent 2,000

migratory birds (not endangered species) from dying in oil-filled

ponds was as great as that for preventing 20,000 or 200,000 birds

from dying. Diminishing marginal willingness to pay for

additional protection could be expected to result in some drop. 

But a drop to zero, especially when the willingness to pay for

the first 2,000 birds is certainly not trivial, is hard to

explain as the expression of a consistent, rational set of
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choices.

It has been argued on a more technical level that the

studies finding such apparent inconsistencies are defective, that

the choices are not presented clearly to the respondents. In the

study referred to immediately above, for instance, respondents

were told that 2,000 birds was "...much less than 1%" of the

total migratory bird population while 200,000 birds was

"...about 2%" of the total. This may have led respondents to

evaluate the programs as being essentially the same. But on the

face of it, the evidence certainly raises some serious questions

about the rationality of the responses.

It could be asked whether rationality is indeed needed. Why

not take the values found as given? There are two answers. One

is that we do not know yet how to reason about values without

some assumption of rationality, if indeed it is possible as all.

 Rationality requirements impose a constraint on the possible

values, without which damage judgments would be arbitrary. A

second answer is that, as discussed above, it is difficult to

find objective counterparts to verify the values obtained in

response to questionnaires. Therefore, some form of internal

consistency is the least we would need to feel some confidence

that the verbal answers corresponded to some reality.

Implausibility of Responses

The CV method is generally used to elicit values for a

specific program to prevent environmental damage, whether it be
13

dead animals, spoilage of a pristine wilderness area, or loss of

visibility in some very unusually clear area. Though in each

case, individuals often express zero willingness to pay, average

willingness to pay over the whole sample is often at least a few

dollars and frequently $20 to $50. With 100,000,000 households

in the United States, these responses result in very large

totals, frequently over $1 billion. Some have argued that these

large sums are in themselves incredible and cast doubt on the CV

method. The Panel is not convinced by this argument, since it is

hard to have an intuition as to a reasonable total.

But there is a different problem with these answers. one

can envision many possible types of environmental damage -- oil

spills or groundwater contamination in many different locations,

visibility impairment in a variety of places, and so on. Would

the average individual or household really be willing to pay $50

or even $5 to prevent each one? This seems very unlikely, since

the total resulting willingness to pay for all such programs

could easily become a very large fraction of one's income or

perhaps even exceed it.

In other words, even if the willingness to pay responses to

individual environmental insults are correct if only one program

is to be considered, they may give overestimates when there are

expected to be a large number of environmental problems. 

Similarly, if individuals fail to consider seriously the public

or private goods that might be substitutes for the resources in

question, their responses to questions in a CV survey may be
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unrealistically large.

Absence of a Meaningful Budget Constraint

Even if respondents in CV surveys take seriously the

hypothetical referendum (or other type of) questions being asked

them, they may respond without thinking carefully about how much

disposable income they have available to allocate to all causes,

public and private (see Kemp and Maxwell (1992), for instance). 

Specifically, respondents might reveal a willingness to pay of,

say, $100 for a project that would reduce the risk of an oil

spill; but if asked what current or planned expenditures they

would forgo to pay for the program, they might instead

re-evaluate their responses and revise them downward. This is

similar to the problem identified immediately above where

individuals fail to think of the possible multiplicity of

environmental projects or policies they might be asked to

support. To date, relatively few CV surveys have reminded

respondents convincingly of the very real economic constraints

within which spending decisions must be made.

Information Provision and Acceptance

If CV surveys are to elicit useful information about

willingness to pay, respondents must understand exactly what it

is they are being asked to value (or vote upon) and must accept

the scenario in formulating their responses. Frequently, CV

surveys have provided only sketchy details about the project(s)
15

being valued and this calls into question the estimates derived

therefrom.

Consider the following example. Suppose information is

desired about individuals' willingness to pay to prevent a

chemical leak into a river. Presumably, their responses would

depend importantly on how long it would take for the chemical to

degrade naturally in the river (if it would at all), what

ecological and human health damage the chemical would do until it

had degraded, and so on. Absent information about such matters,

it is unreasonable to expect even very bright and well-informed

respondents to place meaningful values on a program to prevent

leaks.

Even if detailed information were supplied, there are limits

on the ability of respondents to internalize and thus accept and

proceed from the information given. It is one thing to tell

respondents matter-of-factly that complete recovery will occur

in, say, two years. It is another thing for them to accept this

information completely and then incorporate it in their answers

to difficult questions.

To return to the example above, respondents who take a

pessimistic view of the probable consequences of a chemical leak

are likely to report relatively high willingness to pay to

prevent the contamination -- too high, in fact, if in actuality

such an event had less serious effects. On the other hand,

respondents with an exaggerated sense of the river's assimilative

capacity or regenerative power could be expected to report a
16

willingness to pay that understates their "true" valuation if

provided with a more complete description of likely consequences.

To repeat, even when CV surveys provide detailed and

accurate information about the effects of the program being

valued, respondents must accept that information in making their

(hypothetical) choices. If, instead, respondents rely on a set

of heuristics ("these environmental accidents are seldom as bad

as we're led to believe," or "authorities almost always put too

good a face on these things"), in effect they will be answering

a different question from that being asked; thus, the resulting

values that are elicited will not reliably measure willingness

to pay.

Extent of the Market

Suits for environmental damages are brought by trustees on

behalf of a legally definable group. This group limits the

population that is appropriate for determining damages even

though individuals outside of this group may suffer loss of

passive and active use. Undersampling and even zero sampling of

a subgroup of the relevant population may be appropriate if the

subgroup has a predictably low valuation of the resource. For

example, the authors of the CV study conducted in connection with

the Nestucca oil spill limited their sample to households in

Washington and British Columbia possibly because the individuals

living elsewhere were presumed to have values too low to justify

examination (or possibly because the sponsors of the study were
17

agencies of the State of washington and the province of British

Columbia and so defined the legally appropriate population)

(Rowe, Shaw, and Schulze, 1992).

"Warm Glow" Effects

Some critics of the CV technique (e.g., Diamond and Hausman

(1992)) have observed that the distribution of responses to

open-ended questions about willingness to pay often is

characterized by a significant proportion of "zeros" -- people

who would pay nothing for the program -- and also a number of

sizable reports. This bi-modal distribution also characterizes

individual giving: most of us give nothing to most charities,

but give non-trivial amounts to the ones we do support (at least

$10 or $20, say). This has led these critics to conclude that

individuals' responses to CV questions serve the same function as

charitable contributions -- not only to support the organization

in question, but also to feel the "warm glow" that attends

donating to worthy causes (see Andreoni (1989)). If this is so,

CV responses should not be taken as reliable estimates of true

willingness to pay, but rather as indicative of approval for the

environmental program in question.

III. KEY ISSUES IN THE DESIGN OF CONTINGENT VALUATION

INSTRUMENTS

In the course of its deliberations, the Panel discussed many

issues surrounding the design of CV surveys. Here we provide our
18

views on several issues that are especially important. In

Section IV and in an Appendix to this report, we provide much

greater detail on the characteristics of a valid application of

the CV method.

The Referendum Format

Considered as a survey, a CV instrument is descriptive

rather than explanatory. Description may be as simple as

reporting univariate averages of one kind or another, such as the

percentages of those employed, seeking work, and not seeking work

in the United States, the mean number of rooms occupied by

American households, or the proportion of "likely" voters

favoring one or another candidate in an upcoming election. A CV

study seeks to find the average willingness to pay for a specific

environmental improvement. Nevertheless, as will be seen later,

it is often desirable to ask respondents to specify the reasons

for their reported choices.

Univariate descriptive results are meaningful mainly when

the alternative responses to a question are simple and can be

well specified and there is a high consensus among both

respondents and investigators about the precise meaning of the

questions and answers. In some cases where consensus would

initially not be adequate, simple definitions can be added to a

questionnaire to attain satisfactory agreement -- e.g., in asking

people how many rooms they have in their homes, one states

whether bathrooms, basements, etc. are to be included in the
19

count; most respondents will conform to this specification.

With questions about subjective phenomena, such as attitudes

and values, treating answers as simply descriptive is seldom

meaningful. Too much depends on how questions are worded, and

there is neither sufficient social consensus about precise

meaning, nor an external reference to facilitate such consensus.

 There are many examples in the survey literature of how changes

in wording or context will affect results based on questions

about subjective phenomena (see Schuman and Presser (1981)). For

example, in national surveys close to a quarter of the population

will choose the "don't know" response to most attitude questions

if it is explicitly offered; yet these same people will select a

substantive alternative if "don't know" is not specifically

provided, even though accepted when asserted spontaneously. More

puzzlingly, a question about "forbidding" a particular action

tends to elicit less agreement than a question about "not

allowing" the same action, although the two questions are

logically equivalent. Beyond these examples, most attitude

objects are simply too complex to be summarized by a single

survey question, e.g., attitudes toward abortion are too

dependent on the reasons for abortion and the time in pregnancy

to be adequately captured by a single question; attitudes toward

"gun control" vary enormously depending on the exact framing of

the issue (e.g., handguns vs. all guns, registration vs. banning,

and other concrete policy distinctions).

Contingent valuation studies seek descriptive information,
20

yet call for a response similar to those elicited by questions

about subjective phenomena. Thus they risk many of the same

response effects and other wording difficulties that turn up

regularly in attitude surveys. Minimizing these effects presents

a considerable challenge to anyone wishing to elicit reliable CV

estimates. The simplest way to approach the problem is to

consider a CV survey as essentially a self-contained referendum

in which respondents vote on whether to tax themselves or not for

a particular purpose. Since real referenda are exposed to most

of the response effects that occur with attitude surveys, and

since we take the result of referenda as telling us something

about "true" preferences, it is not necessary to claim they can

be eliminated completely in a CV study.

 The Panel is of the opinion that open-ended CV questions --

e.g., "What is the smallest sum that would compensate you for

environmental damage X?" or, "What is the largest amount you

would be willing to pay to avoid (or repair) environmental damage

X?" -- are unlikely to provide the most reliable valuations. 

There are at least two reasons for this conclusion. In the first

place, the scenario lacks realism since respondents are rarely

asked or required in the course of their everyday lives to place

a dollar value on a particular public good. Their responses to

such questions are therefore likely to be unduly sensitive to

trivial characteristics of the scenario presented. In the second

place, an open-ended request for willingness to pay or

willingness to accept compensation invites strategic
21

overstatement. The more seriously the respondent takes the

question, the more likely it is that he or she will see that

reporting a large response is a costless way to make a point. 

Both experience and logic suggest that responses to open-ended

questions will be erratic and biased.

However, the referendum format, especially when cast in the

willingness to pay mode -- "Would you be willing to contribute

(or be taxed) D dollars to cover the cost of avoiding or

repairing environmental damage X?" -- has many advantages. It is

realistic: referenda on the provision of public goods are not

uncommon in real life. There is no strategic reason for the

respondent to do other than answer truthfully, although a

tendency to overestimate often appears even in connection with

surveys concerning routine market goods. The fact that market

surveys continue to be used routinely suggests that this tendency

is not a insuperable obstacle. Of course, the respondent in a CV

survey understands that the referendum is hypothetical; there is

no implication that the tax will actually be levied and the

damage actually repaired or avoided. This suggests that

considerable efforts should be made to induce respondents to take

the question seriously, and that the CV instrument should contain

other questions designed to detect whether the respondent has

done so. Although Carson, et al. (1992), included a useful

question to determine whether respondents believed the survey was

biased in any direction, they did not sufficiently test whether

the completeness of, and time period for, restoration stated in
22

the survey were fully accepted by respondents. But, as far as

strategic reasons go, a respondent who would not be willing to

pay D dollars has no reason to answer "Yes," and a respondent who

would be willing to pay D dollars has no reason to answer "No."

There are, however, several other reasons why one's response

 to a hypothetical referendum question might be the opposite of

one's actual vote on a real ballot. On one hand, a respondent

unwilling to pay D dollars in reality might feel pressure to give

the "right" or "good" answer when responding to an in-person or

telephone interviewer. This could happen if the respondent

believes that the interviewer would herself favor a yes answer. 

On the other hand, a respondent actually willing to pay the

stated amount might answer in the negative for several reasons:

(i) belief that the proposed scenarios distributed the burden

unfairly; (ii) doubt of either the feasibility of the proposed

action, so that any contribution would be wasted, or the ability

of the relevant agency to carry out the action efficiently; or

(iii) refusal to accept the hypothetical choice problem, because

of either a generalized aversion to taxes or a view that someone

else -- the "oil industry", for example -- should pay for repair

or avoidance as the responsible party. The same considerations

suggest that a CV instrument should include questions designed to

detect the presence of these sources of bias. This is in fact

often done, but we do not know how successfully. 

There are two further problems that could detract from the

reliability of CV responses without producing any determinate
23

bias: (i) a feeling that one's vote will have no significant

effect on the outcome of the hypothetical referendum, leading to

no reply or an unconsidered one; and (ii) poor information about

the damage being valued. Of course, either of these could occur

in real referenda.

Here we must decide on the standard of knowledgeability of

the respondents that we want to impose on a CV study. It is

clear that it should be at least as high as that which the

average voter brings to a real referendum on the provision of a

specific public good, but should it be higher? A "conservative"

CV study, i.e., one that avoids overestimating true willingness

to pay, will no doubt exceed the minimum standard of information

and will also lean over backwards to avoid providing information

in a way that might bias the response upwards. In particular, a

conservative study will provide the respondent with some

perspective concerning the overall frequency and magnitude of oil

spills, the amount of money currently being spent on preventing

and remedying them, the overall scale of their consequences, the

peculiar features of the spill in question, and similar relevant

information. Placing the choice problem in a broader context

helps the respondent to arrive at a realistic or even

conservative valuation.

Most of the provision of public goods in this country is

decided by representatives and bureaucrats rather that by direct

vote of the citizens. It is presumed that these agents are more

"expert" or at least draw on more knowledge than the citizens
24

themselves. The agents' expertise, if it really exists, is about

the means and cost of providing public goods, though elected

officials may sometimes be presumed to "represent" judgments of

ultimate value to the citizens. Nevertheless, to increase one's

confidence that a CV study is conservatively reliable, one might

want to compare its outcome with that provided by a panel of

experts. This will help check whether respondents and those

conducting the study or studies are reasonably well-informed and

well-motivated. This comparison could be made on a sample of CV

studies to give an idea of their reliability in general.

The above considerations suggest that a CV study based on

the referendum scenario can produce more reliably conservative

estimates of willingness to pay, and hence of compensation

required in the aftermath of environmental impairment, provided

that a concerted effort is made to motivate the respondents to

take the study seriously, to inform them about the context and

special circumstances of the spill or other accident, and to

minimize any bias toward high or low answers originating from

social pressure within the interview. This implies that, in the

present state of the art, a reliably conservative CV study should

be conducted with personal interviews of significant duration and

will therefore be relatively costly. If follows therefore that,

in order that the cost of the study not be disproportionately

large compared to the amount of damages, the CV approach would

likely be used only in relatively major spills, at least until

further improvements in methodology can be developed and
25

accepted. (A suggestion for doing so is offered in Section V.)

 The referendum format offers one further advantage for CV.

 As we have argued, external validation of elicited lost passiveuse values is usually impossible. There are however real-life

referenda. Some of them, at least, are decisions to purchase

specific public goods with defined payment mechanisms, e.g., an

increase in property taxes. The analogy with willingness to pay

for avoidance or repair of environmental damage is far from

perfect but close enough that the ability of CV-like studies to

predict the outcomes of real-world referenda would be useful

evidence on the validity of the CV method in general. 

The test we envision is not an election poll of the usual

type. Instead, using the referendum format and providing the

usual information to the respondents, a study should ask whether

they are willing to pay the average amount implied by the actual

referendum. The outcome of the CV-like study should be compared

with that of the actual referendum. The Panel thinks that

studies of this kind should be pursued as a method of validating

and perhaps even calibrating applications of the CV method (see

Magleby, 1984).

Addressing the Embedding Problem

Perhaps the most important internal argument against the

reliability of the CV approach (as against general criticisms

about vagueness, lack of information, or unreality of the

scenario) is the observation of the "embedding" phenomenon (see
26

the discussion in Section II). Different but similar samples of

respondents are asked about their willingness to pay for

prevention of environmental damage scenarios that are identical

except for their scale: different numbers of seabirds saved,

different numbers of forest tracts preserved from logging, etc. 

It is reported that average willingness to pay is often

substantial for the smallest scenario presented but is then

substantially independent of the size of the damage averted,

rising slightly if at all for large changes in size.

The usual interpretation proposed by critics of the CV

method is that the responses are not measuring the equivalent

dollar value of the utility of the environmental assets

preserved, because that would certainly be measurably larger for

substantially larger programs of preservation. Instead, the

fixed sum offered is the value of a feeling of having done

something praiseworthy; a "warm glow" is the phrase often used.

This is potentially a very damaging criticism of the method.

 CV studies almost always seek to measure willingness to pay to

avoid a particular incident rather than compensation that would

be required for damage that has already occurred. This is

because respondents are more likely to exaggerate the

compensation they would require than their willingness to pay,

and because the latter is expected to be less than the former and

so is conservative. If reported willingness to pay accurately

reflected actual willingness to pay, then, under the "warm glow"

interpretation, willingness to pay might well exceed compensation
27

 required because the former contains an element of selfapprobation. It might be real but not properly compensable. 

 Defenders of the CV approach reply to this criticism in

various ways. Sometimes it is argued that the evidence used to

support "embedding" simply indicates diminishing marginal utility

of the asset in question. In many cases, however, the constancy

or near-constancy of willingness to pay does not appear

consistent with the large reported amounts for the first small

increment of environmental preservation.

A second defense of CV against the embedding phenomenon is

that CV questions have to be posed carefully and in context. It

is argued that carelessly formulated CV instruments leave

respondents with the impression that they are being asked, "Would

you pay $X to avert a certain small environmental harm?" In a

very large population of birds, the death of 1,000 is not seen as

noticeably different from the death of 100,000 -- and may not

actually be very different -- so that respondents simply answer

the question just asked.

This second response leads to the obvious question: how

should a CV instrument be framed to elicit an answer that

responds to the precise scenario and not to a generalized "warm

glow" effect? We must reject one possible approach, that of

asking each respondent to express willingness to pay to avert

incidents of varying sizes; the danger is that embedding will be

forcibly avoided, still without realism. This issue is best

considered as part of the broader question: How much context
28

about the incident itself and about the respondent's

circumstances and choices should be included in the CV

instrument?

We are recommending a high standard of richness in context

to achieve a realistic background. Our proposed guidelines

regarding this issue are embodied in Section IV below.

Time Dimension of Passive Use Losses

Typically, environmental damages from oil spills or similar

accidents are severe for some period of time -- weeks, months, or

sometimes a few years -- and gradually are reduced by natural

forces and human efforts to a low or possibly even zero steady

state level. In some circumstances, passive-use losses derive

only or mostly from the steady state conditions; thus, if passive

use value derives from species diversity, even a considerable

loss of birds or mammals which does not endanger any species will

give rise to no loss. If, on the contrary, considerable passiveuse value is attached to the interim state of the natural

resource, then respondents have to do a very difficult present

value calculation properly to compute their current willingness

to pay for the difference between the fully restored state of the

resource and the actual state as the level of restoration varies

over time. CV surveys accordingly have to be carefully designed

to allow respondents to differentiate interim from steady state

passive-use loss, and, if there is interim passive-use loss, to

report its present value correctly.
29

It is reasonable to assume that interim passive-use values

are additive over time. Hence, we need a calculation of present

values of the interim losses. The discounting and the estimation

of the rate of recovery of the resource should be done by

technical experts and not by the respondents, who are unlikely to

handle these tasks adequately. Respondents should be asked only

their willingness to pay to eliminate the difference between some

partially restored level of the resource and the pristine state

for a specific period of time, say a year, on the assumption that

after that time full restoration is assured. Technical experts

would estimate how the state of the resource will vary from year

to year as the restoration takes place. The technical

information about the state of the resource, together with the

respondent's assessments of the flow valuation of the resource,

can be used to construct a time series of passive-use losses

which can be discounted to the present at an appropriate rate of

interest to determine the present value of the damages.

IV. SURVEY GUIDELINES

In this section we try to lay down a fairly complete set of

guidelines compliance with which would define an ideal CV survey.

 A CV survey does not have to meet each of these guidelines fully

in order to qualify as a source of reliable information to a

damage assessment process. Many departures from the guidelines

or even a single serious deviation would, however, suggest

unreliability prima facie. To preserve continuity, we give only
30

a bald list of guidelines here. They are repeated together with

further explanatory comments in the Appendix to this Report.

GENERAL GUIDELINES

0 Sample Type and Size: Probability sampling is essential for

a survey used for damage assessment.1 The choice of sample

specific design and size is a difficult, technical question

that requires the guidance of a professional sampling

statistician.

0 Minimize Nonresponses: High nonresponse rates would make

the survey results unreliable.

0 Personal Interview: The Panel believes it unlikely that

reliable estimates of values could be elicited with mail

surveys. Face-to-face interviews are usually preferable,

although telephone interviews have some advantages in terms

of cost and centralized supervision.

0 Pretesting for Interviewer Effects: An important respect in

 

1 This need not preclude use of less adequate samples,

including quota or even convenience samples, for preliminary

testing of specific experimental variations, so long as order of

magnitude differences rather than univariate results are the focus.

 Even then, obvious sources of bias should be avoided (e.g.,

college students are probably too different in age and education

from the heterogeneous adult population to provide a trustworthy

basis for wider generalization).
31

which CV surveys differ from actual referenda is the

presence of an interviewer (except in the case of mail

surveys). It is possible that interviewers contribute to

"social desirability" bias, since preserving the environment

is widely viewed as something positive. In order to test

this possibility, major CV studies should incorporate

experiments that assess interviewer effects.

0 Reporting: Every report of a CV study should make clear the

definition of the population sampled, the sampling frame

used, the sample size, the overall sample non-response rate

and its components (e.g., refusals), and item non-response

on all important questions. The report should also

reproduce the exact wording and sequence of the

questionnaire and of other communications to respondents

(e.g., advance letters). All data from the study should be

archived and made available to interested parties (see

Carson et al. (1992), for an example of good practice in

inclusion of questionnaire and related details; as of this

date, however, the report has not been available publicly

and the data have not been archived for open use by other

scholars).

0 Careful Pretesting of a CV Questionnaire: Respondents in a

CV survey are ordinarily presented with a good deal of new

and often technical information, well beyond what is typical
32

in most surveys. This requires very careful pilot work and

pretesting, plus evidence from the final survey that

respondents understood and accepted the main description and

questioning reasonably well.

GUIDELINES FOR VALUE ELICITATION SURVEYS

The following guidelines are met by the best CV surveys and

need to be present in order to assure reliability and usefulness

of the information that is obtained.

0 Conservative Design: Generally, when aspects of the survey

design and the analysis of the responses are ambiguous, the

option that tends to underestimate willingness to pay is

preferred. A conservative design increases the reliability

of the estimate by eliminating extreme responses that can

enlarge estimated values wildly and implausibly.

0 Elicitation Format: The willingness to pay format should be

used instead of the compensation required because the former

is the conservative choice.

0 Referendum Format: The valuation question should be posed

as a vote on a referendum.

0 Accurate Description of the Program or Policy: Adequate
33

information must be provided to respondents about the

environmental program that is offered. It must be defined

in a way that is relevant to damage assessment.

0 Pretesting of Photographs: The effects of photographs on

subjects must be carefully explored.

0 Reminder of Undamaged Substitute Commodities: Respondents

must be reminded of substitute commodities, such as other

comparable natural resources or the future state of the same

natural resource. This reminder should be introduced

forcefully and directly prior to the main valuation question

to assure that respondents have the alternatives clearly in

mind.

0 Adequate Time Lapse from the Accident: The survey must be

conducted at a time sufficiently distant from the date of

the environmental insult that respondents regard the

scenario of complete restoration as plausible. Questions

should be included to determine the state of subjects'

beliefs regarding restoration probabilities.

0 Temporal Averaging: Time dependent measurement noise should

be reduced by averaging across independently drawn samples

taken at different points in time. A clear and substantial
34

time trend in the responses would cast doubt on the

"reliability" of the finding.

0 "No-answer" Option: A "no-answer" option should be

explicitly allowed in addition to the "yes" and "no" vote

options on the main valuation (referendum) question. 

Respondents who choose the "no-answer" option should be

asked nondirectively to explain their choice. Answers

should be carefully coded to show the types of responses,

for example: (i) rough indifference between a yes and a no

vote; (ii) inability to make a decision without more time or

more information; (iii) preference for some other mechanism

for making this decision; and (iv) bored by this survey and

anxious to end it as quickly as possible.

0 Yes/no Follow-ups: Yes and no responses should be followed

up by the open-ended question: "Why did you vote yes/no?" 

Answers should be carefully coded to show the types of

responses, for example: (i) It is (or isn't) worth it; 

(ii) Don't know; or (iii) The oil companies should pay.

0 Cross-tabulations: The survey should include a variety of

other questions that help to interpret the responses to the

primary valuation question. The final report should include

summaries of willingness to pay broken down by these

categories. Among the items that would be helpful in
35

interpreting the responses are:

Income

Prior Knowledge of the Site

Prior Interest in the Site (Visitation Rates)

Attitudes Toward the Environment

Attitudes Toward Big Business

Distance to the Site

Understanding of the Task

Belief in the Scenarios

Ability/Willingness to Perform the Task

0 Checks on Understanding and Acceptance: The above

guidelines must be satisfied without making the instrument

so complex that it poses tasks that are beyond the ability

or interest level of many participants.

GOALS FOR VALUE ELICITATION SURVEYS

The following items are not adequately addressed by even the

best CV surveys. In the opinion of the Panel, these issues will

need to be convincingly dealt with in order to assure the

reliability of the estimates.

0 Alternative Expenditure Possibilities: Respondents must be

reminded that their willingness to pay for the environmental

program in question would reduce their expenditures for

private goods or other public goods. This reminder should
36

be more than perfunctory, but less than overwhelming. The

goal is to induce respondents to keep in mind other likely

expenditures, including those on other environmental goods,

when evaluating the main scenario.

0 Deflection of Transaction Value: The survey should be

designed to deflect the general "warm-glow" of giving or the

dislike of "big business" away from the specific

environmental program that is being evaluated. It is

possible that the referendum format limits the "warm glow"

effect, but until this is clear the survey design should

explicitly address this problem.

0 Steady State or Interim Losses: It should be made apparent

that respondents can distinguish interim from steady-state

losses.

0 Present Value Calculations of Interim Losses: It should be

demonstrated that, in revealing values, respondents are

adequately sensitive to the timing of the restoration

process.

0 Advance Approval: Since the design of the CV survey can

have a substantial effect on the responses, it is desirable

that -- if possible -- critical features be preapproved by
37

both sides in a legal action, with arbitration and/or

experiments used when disagreements cannot be resolved by

the parties themselves.

0 Burden of Proof: Until such time as there is a set of

reliable reference surveys, the burden of proof of

reliability must rest on the survey designers. They must

show through pretesting or other experiments that their

survey does not suffer from the problems that these

guidelines are intended to avoid. Specifically, if a CV

survey suffered from any of the following maladies, we would

judge its findings "unreliable":

-- A high nonresponse rate to the entire survey

instrument or to the valuation question.

-- Inadequate responsiveness to the scope of the

environmental insult.

-- Lack of understanding of the task by the

respondents.

-- Lack of belief in the full restoration scenario.

-- "Yes" or "no" votes on the hypothetical referendum

that are not followed up or explained by making
38

reference to the cost and/or the value of the

program.

0 Reliable Reference Surveys: In order to alleviate this

heavy burden of proof, we strongly urge the government to

undertake the task of creating a set of reliable reference

surveys that can be used to interpret the guidelines and

also to calibrate surveys that do not fully meet the

conditions.

V. RECOMMENDATIONS FOR FUTURE RESEARCH

The Panel's major research recommendation goes toward a

drastic reform of the CV procedure, extending beyond the

guidelines suggestion in Section IV.

The problem of estimating the demand for highly innovative

commercial products, including some that have not yet actually

been produced, is much like the problem faced in CV research. It

is the problem of estimating willingness to pay for a necessarily

unfamiliar product. The field of market research has developed

methods -- "conjoint analysis," for example -- that are very

similar to the CV approach. (One important difference is that a

new product may eventually reach the market, and projections of

expected sales can be checked. Survey responses are usually

found to be moderate overestimates of actual willingness to pay.)

 Practitioners have found that survey methods are better at

estimating relative demand than absolute demand. There is an
39

anchoring problem, even with private goods -- that is, absolute

willingness to pay is hard to pin down. This leads to the

following suggestion.

The federal government should produce standard damage

assessments for a few specific reference oil spills, either

hypothetical or actual, ranging from small to large. These

standard valuations could be generated by any method. One

possibility would be through a jury of experts. Such a jury of

experts might wish to conduct a series of CV studies, satisfying

the guidelines laid out above. These CV studies would be inputs

into the jury process, to be combined with other information and

expert judgment. Once these benchmarks were available, they

could serve as reference points for later CV studies. When a

damage assessment is required, surveys could be used to elicit

answers to questions like: "Would you pay (much more, more,

about the same, less, much less) to prevent this spill than you

would to prevent Standard Spill A?" "Would you pay an amount to

avoid this spill that is between the amounts you would pay to

avoid Standard Spill B and Standard Spill C? If so, is the

amount much closer to B than C, closer to B than C, halfway

between B and C, closer to C than B, much closer to C than B?" 

These questions presumably would not be asked so schematically. 

Responses to such a study could then serve as one reliable source

of information in the damage assessment.

We recognize that this technique would require that

respondents be made familiar with the reference spills as well as
40

the particular spill whose damage is being assessed. We expect

that the additional effort would be more than offset by the

greater simplicity and reliability in estimating relative

willingness to pay.

This possibility suggests a slightly more radical extension

of the CV method. Respondents could be asked to compare their

willingness to pay to avoid a specific case of environmental

damage to their willingness to pay for a range of fairly familiar

private goods. It would no doubt be best if the private goods

were to bear some similarity to the environmental good in

question, but that is not necessary. The anchoring purpose would

be served if respondents could measure their willingness to pay

in units of articles of clothing or small household appliances

forgone.

This latter is a suggestion for research in the CV method,

not necessarily a recommendation for current practical use.

The guidelines proposed in Section IV themselves suggest

areas for further research, this time within the contingent

valuation community. In particular, we emphasize the urgency of

studying the sensitivity of willingness to pay responses to the

number and extent of budgetary substitutes mentioned in survey

instruments (that is, reminders of other things on which

respondents could spend their money). In such research it would

be helpful if parallel studies were conducted on the sensitivity

of stated intentions to buy ordinary market goods -- both

familiar and unfamiliar -- to reminders of alternative uses of
41

those resources. The point is to discover the extent to which

the valuation of environmental public goods is intrinsically more

difficult than similar exercises with respect to market goods.

A closely-related line of research is the sensitivity of

responses in CV surveys to the number and extent of undamaged

substitute commodities mentioned explicitly in the survey

instrument (miles of nearby shoreline, miles of shoreline

elsewhere, similarity for animal or bird life, alternative

recreation possibilities and so on). This could be extended to

variations in the way in which the budget constraint is presented

to respondents. Here again, comparisons with market goods would

be useful.

Finally, having urged that the availability of a no-vote

option is an important component of the ability of the CV

technique to mimic an actual referendum, we recommend further

research into alternative ways of presenting and interpreting the

no-vote option. In this respect, too, comparative studies with

familiar public and private goods (local parks, school

facilities, housing for the homeless, food distributions) would

be enlightening. Real referenda always allow the option of not

voting, in a natural way. CV studies have to achieve the same

result more deliberately, so there is a need to know if the

precise formulation matters very much to the result.

VI. CONCLUSIONS AND RECOMMENDATIONS

The Panel starts from the premise that passive-use loss --
42

interim or permanent -- is a meaningful component of the total

damage resulting from environmental accidents. A problem arises

because passive-use losses have few or no overt behavioral

consequences. The faintness of the behavioral trail means that a

well-designed and adequately sensitive measuring instrument is

needed to substitute for conventional observations of behavior. 

In particular, can the CV method provide a sufficiently reliable

estimate of total loss -- including passive-use loss -- to play a

useful role in damage assessment?

It has been argued in the literature and in comments

addressed to the Panel that the results of CV studies are

variable, sensitive to details of the survey instrument used, and

vulnerable to upward bias. These arguments are plausible. 

However, some antagonists of the CV approach go so far as to

suggest that there can be no useful information content to CV

results. The Panel is unpersuaded by these extreme arguments.

In Section IV above, we identify a number of stringent

guidelines for the conduct of CV studies. These require that

respondents be carefully informed about the particular

environmental damage to be valued, and about the full extent of

substitutes and undamaged alternatives available. In willingness

to pay scenarios, the payment vehicle must be presented fully and

clearly, with the relevant budget constraint emphasized. The

payment scenario should be convincingly described, preferably in

a referendum context, because most respondents will have had

experience with referendum ballots with less-than-perfect
43

background information. Where choices in formulating the CV

instrument can be made, we urge they lean in the conservative

direction, as a partial or total offset to the likely tendency to

exaggerate willingness to pay.

The Panel concludes that under those conditions (and others

specified above), CV studies convey useful information. We think

it is fair to describe such information as reliable by the

standards that seem to be implicit in similar contexts, like

market analysis for new and innovative products and the

assessment of other damages normally allowed in court

proceedings. As in all such cases, the more closely the

guidelines are followed, the more reliable the result will be. 

It is not necessary, however, that every single injunction be

completely obeyed; inferences accepted in other contexts are not

perfect either.

Thus, the Panel concludes that CV studies can produce

estimates reliable enough to be the starting point of a judicial

process of damage assessment, including lost passive-use values.

 To be acceptable for this purpose, such studies should follow

the guidelines described in Section IV above. The phrase "be the

starting point" is meant to emphasize that the Panel does not

suggest that CV estimates can be taken as automatically defining

the range of compensable damages within narrow limits. Rather,

we have in mind the following considerations.

The Panel is persuaded that hypothetical markets tend to

overstate willingness to pay for private as well as public goods.
44

 The same bias must be expected to occur in CV studies. To the

extent that the design of CV instruments makes conservative

choices when alternatives are available, as urged in Section IV,

this intrinsic bias may be offset or even over-corrected. All

surveys of attitudes or intentions are bound to exhibit

sensitivity of response to the framing of questions and the order

in which they are asked. No automatic or mechanical calibration

of responses seems to be possible. 

The judicial process must in each case come to a conclusion

about the degree to which respondents have been induced to

consider alternative uses of funds and take the proposed payment

vehicle seriously. Defendants will argue that closer attention

to substitute commodities would have yielded lower valuations. 

Trustees will argue that they have already leaned over backwards

to ensure conservative responses. Judges and juries must decide

as they do in other damage cases. The Panel's conclusion is that

a well-conducted CV study provides an adequately reliable

benchmark to begin such arguments. It contains information that

judges and juries will wish to use, in combination with other

evidence, including the testimony of expert witnesses.

The Panel's second conclusion is that the appropriate

federal agencies should begin to accumulate standard damage

assessments for a range of oil spills, as described in Section V.

 That process should further improve the reliability of CV

studies in damage assessment. It should thus contribute to

increasing the accuracy and reducing the cost of subsequent
45

damage assessment cases. In that sense, it can be regarded as an

investment.

The proposals for further research outlined in Section V are

an integral part of our recommendations. The Panel believes that

the suggestions put forward there could lead to more reliable and

less controversial damage assessment at reduced cost. It is not

to be expected that controversy will disappear, however. There

will always be controversy where intangible losses have to be

evaluated in monetary terms.
46

APPENDIX

GENERAL GUIDELINES

0 Sample Type and Size: Probability sampling is essential for

a survey used for damage assessment.2 The choice of sample

specific design and size is a difficult, technical question

that requires the guidance of a professional sampling

statistician.

If a single dichotomous question of the yes-no type is used

to elicit valuation responses, then a total sample size of 1000

respondents will limit sampling error to about 3% plus or minus

on a single dichotomous question, assuming simple random

sampling. However, this or any other sample size needs to be

reconceptualized for three reasons. First, if face-to-face

interviewing is used, as we suggest above, clustering and

stratification must be taken into account. Second, if

dichotomous valuation questions are used (e.g., hypothetical

referenda), separate valuation amounts must be asked of random

sub-samples and these responses must be unscrambled

econometrically to estimate the underlying population mean or

median. Third, in order to incorporate experiments on

 

2 This need not preclude use of less adequate samples,

including quota or even convenience samples, for preliminary

testing of specific experimental variations, so long as order of

magnitude differences rather than univariate results are the focus.

 Even then, obvious sources of bias should be avoided (e.g.,

college students are probably too different in age and education

from the heterogeneous adult population to provide a trustworthy

basis for wider generalization).
47

interviewer and wording effects, additional random sub-sampling

is required. For all these reasons, it will be important to

consult sampling statisticians in the design of a CV survey

intended for legal or policy-making purposes.

0 Minimize Nonresponses: High nonresponse rates would make

the survey results unreliable.

To the extent that a CV study is expected to represent the

adult population of the United States or a portion of it,

minimizing both sample non-response and item non-response are

important. The former is unlikely to be below 20% even in very

high quality surveys; the latter has also been large in some CV

surveys because of the difficulty of the task respondents are

being asked to perform. These sources of potential bias can be

partially justified on the grounds that they also occur with

official referenda, in both cases with the loss especially of the

least educated parts of the population. The further reduction of

the final sample by elimination of "protest zeros," "unrealistic

high values," and other problematic responses may lead to

effective final total response rates so low as to imply that the

survey population consists of interested and specially instructed

quasi-experts. This consideration reinforces the desirability of

combining a reasonable response rate with a high but not

forbidding standard of information, as discussed in Section III

above. 
48

0 Personal Interview: The Panel believes it unlikely that

reliable estimates of values could be elicited with mail

surveys. Face-to-face interviews are usually preferable,

although telephone interviews have some advantages in terms

of cost and centralized supervision.

Assuming a CV survey is to represent a natural population,

such as all adults in the United States, or those in a single

urban area or a state, it is desirable that it be carried out

using either face-to-face or telephone interviews. Mail surveys

typically employ lists that cover too small a part of the

population (e.g., samples based on telephone directories omit

approximately half the U.S. population because of non-listed

numbers, incorrect numbers, and non-phone households), and then

miss another quarter or more of the remainder through nonresponse. In addition, since the content of a mail questionnaire

can be reviewed by targeted respondents before deciding to return

it, those most interested in a natural resource issue or in one

side or the other can make their decision on that basis. It is

also impossible using mail surveys to guarantee random selection

within households or to confine answering to a single respondent,

and it is difficult (though not impossible) to control questionorder effects. Thus, mail surveys should be used only if another

supplementary method can be employed to cross-validate the

results on a random sub-sample of respondents.
49

The choice between telephone and face-to-face administration

is less clear. Face-to-face surveys offer practical advantages

in maintaining respondent motivation and allowing use of graphic

supplements. Both coverage and response rates are also usually

somewhat higher than with telephone surveys. However, telephone

surveys can cut interviewing costs by between a third and a half;

for CV purposes, it may be a disadvantage that most survey

investigators believe telephone interviews need to be kept

shorter in length than face-to-face interviews because respondent

attention and cooperation are more difficult to maintain. In

addition, random-digit-dial telephone surveys approximate simple

random sampling. Face-to-face surveys must be based on cluster

sampling and, therefore, the results provide less precise

estimates than do telephone surveys of the same size.

0 Pretesting for Interviewer Effects: An important respect in

which CV surveys differ from actual referenda is the

presence of an interviewer (except in the case of mail

surveys). It is possible that interviewers contribute to

"social desirability" bias, since preserving the environment

is widely viewed as something positive. In order to test

this possibility, major CV studies should incorporate

experiments that assess interviewer effects.

To test for interviewer effects, two modifications might be

made to a standard face-to-face CV survey. In one variant on
50

current practice, respondents would stop when they come to the

valuation question, write their "vote" on a ballot, and fold and

deposit it in a sealed box. However, since this practice would

not mimic the complete anonymity of the voting booth, for a subsample of respondents a second modification should be made. 

Respondents would be allowed to mail their "ballots" in unmarked

envelopes directly to the survey organization, even though that

will preclude any but the simplest analysis of responses. Tests

of the effect of both these modifications of current practice

will indicate whether they are needed routinely or whether at

least some calibration should be introduced to compensate for

interviewer effects. (The more modest of these proposed

modifications -- a simulated ballot box, or even voting on a

portable computer -- has few if any disadvantages and might be

made standard if it shows any reliable departure at all from

answers given orally to the interviewer.)

0 Reporting: Every report of a CV study should make clear the

definition of the population sampled, the sampling frame

used, the sample size, the overall sample non-response rate

and its components (e.g., refusals), and item non-response

on all important questions. The report should also

reproduce the exact wording and sequence of the

questionnaire and of other communications to respondents

(e.g., advance letters). All data from the study should be

archived and made available to interested parties (see
51

Carson et al. (1992), for an example of good practice in

inclusion of questionnaire and related details; as of this

date, however, the report has not been available publicly

and the data have not been archived for open use by other

scholars).

0 Careful Pretesting of a CV Questionnaire: Respondents in a

CV survey are ordinarily presented with a good deal of new

and often technical information, well beyond what is typical

in most surveys. This requires very careful pilot work and

pretesting, plus evidence from the final survey that

respondents understood and accepted the main description and

questioning reasonably well.

Parenthetically, the claim sometimes made by CV proponents

that particular methods of piloting, such as focus groups, are

essential should be viewed with skepticism, since these claims

are unsupported by any systematic evidence. Nor is it clear that

what are called "state-of-the-art" CV surveys constitute

something entirely new or different from other types of serious

survey investigations. Thus, although evidence that

questionnaire development has been carried out carefully is

certainly important, it cannot be taken as a self-sufficient

basis of validity -- the more so because we know that many people

will answer survey questions without apparent difficulty, even

when they do not understand them well. A way of reducing
52

pressure to give answers of questionable meaningfulness would be

to provide respondents an explicit "no opinion" type of

alternative when a key valuation question is posed.

GUIDELINES FOR VALUE ELICITATION SURVEYS

The following guidelines are met by the best CV surveys and

need to be present in order to assure reliability and usefulness

of the information that is obtained.

0 Conservative Design: Generally, when aspects of the survey

design and the analysis of the responses are ambiguous, the

option that tends to underestimate willingness to pay is

preferred. A conservative design increases the reliability

of the estimate by eliminating extreme responses that can

enlarge estimated values wildly and implausibly.

0 Elicitation Format: The willingness to pay format should be

used instead of compensation required because the former is

the conservative choice.

In experimental settings, the gap between stated intentions

to support a particular referendum and actual behavior in the

voting booth can be very great (see Magleby, 1984). This gap

might be treated by "calibration" if there were historical data

on the relationship between such intentions and behavior. 

Unfortunately, we are aware of no data that is close enough to
53

the CV context that could be used to calibrate CV responses. In

the absence of historical data that can be used to calibrate the

intentions reported in the CV surveys, the survey instrument has

to be designed with extraordinary care so that it can stand on

its own.

0 Referendum Format: The valuation question should be posed

as a vote on a referendum.

As is now generally recognized by most CV proponents, asking

respondents to give a dollar valuation in response to an openended question presents them with an extremely difficult task. 

At the same time, CV proponents also recognize that presenting

respondents a set of dollar amounts from which they are to choose

is likely to create anchoring and other forms of bias. Thus, we

recommend as the most desirable form of CV elicitation the use of

a dichotomous question that asks respondents to vote for or

against a particular level of taxation, as occurs with most real

referenda. As already noted, such a question form also has

advantage in terms of incentive compatibility. (If a doublebounded dichotomous choice or some other question form is used in

order to obtain more information per respondent, experiments

should be developed to investigate biases that may be

introduced.)

0 Accurate Description of the Program or Policy: Adequate
54

information must be provided to respondents about the

environmental program that is offered. It must be defined

in a way that is relevant to damage assessment.

Ideally a CV survey would elicit attitudes toward three

alternative (future) recovery scenarios: (A) "immediate"

restoration, (b) accelerated restoration, and (c) natural

restoration. Damages would be the difference between (a) and (b)

on the assumption that accelerated restoration is provided by the

responsible party. Unfortunately, respondents may not find

"immediate" restoration very plausible and they may resist the

notion that they should be expected to contribute to accelerated

restoration when it is an oil company that is at fault. If

respondents are unable or unwilling to deal hypothetically with

the most relevant "clean-up" scenarios, alternative "prevention"

scenarios will have to be used in the survey instrument. For

example, respondents may be asked to vote for a referendum that

offers reduced risk of another spill for a specified period of

time.3 The weaker is the linkage between the "prevention"

scenarios and the "clean-up" scenarios, the more unreliable are

the survey results. Rhetorically: Is a decade of prevention

equal in value to the difference in value between accelerated and

immediate clean-up?

 

3 As in the survey actually performed by the State of Alaska

after the Valdez spill (See Carson et al. (1992)).
55

0 Pretesting of Photographs: The effects of photographs on

subjects must be carefully explored.

One effective means for conveying information and holding

interest in a CV interview has been the use of large and

impressive photographs. However, this technique is a two-edged

sword because the dramatic nature of a photograph may have much

more emotional impact than the rest of the questionnaire. Thus

it is important that photographs be subjected to even more

careful assessment than verbal material if the goal is to avoid

bias in presentation.4

0 Reminder of Undamaged Substitute Commodities: Respondents

must be reminded of substitute commodities, such as other

comparable natural resources or the future state of the same

natural resource. This reminder should be introduced

forcefully and directly prior to the main valuation question

to assure that respondents have the alternatives clearly in

mind.

0 Adequate Time Lapse from the Accident: The survey must be

conducted at a time sufficiently distant from the date of

the environmental insult that respondents regard the

scenario of complete restoration as plausible. Questions

 

4 Failure to test the effects of photographs on responses is

one shortcoming of Carson et al. (1992).
56

should be included to determine the state of subjects'

beliefs regarding restoration probabilities.

Survey respondents who would not suffer interim passive-use

loss may not regard full restoration as very plausible;

therefore, they may report substantial passive-use loss even if

told that full restoration in some reasonable amount if time is

certain. Misunderstanding of the restoration probability is most

acute when the accident has recently occurred and before any

substantial restoration takes place. It would be ideal to assess

steady state passive-use loss after natural and human restoration

is complete or nearly so, since then presumably respondents would

believe in the restoration. If that is not a possibility,

surveys might be conducted over time until the reported

willingness to pay settles down (assuming that it does), as the

respondents come to believe more and more in the probable success

of the restoration effort. Alternatively, respondents might be

asked to value a menu of alternative possible scenarios, without

being told explicitly which is applicable for the environmental

insult under study. The menu should be designed to force them to

consider the difference between interim and steady-state passiveuse value.

0 Temporal Averaging: Time dependent measurement noise should

be reduced by averaging across independently drawn samples

taken at different points in time. A clear and substantial
57

time trend in the responses would cast doubt on the

"reliability" of the finding.

0 "No-answer" Option: A "no-answer" option should be

explicitly allowed in addition to the "yes" and "no" vote

options on the main valuation (referendum) question. 

Respondents who choose the "no-answer" option should be

asked nondirectively to explain their choice. Answers

should be carefully coded to show the types of responses,

for example: (i) rough indifference between a yes and a no

vote; (ii) inability to make a decision without more time or

more information; (iii) preference for some other mechanism

for making this decision; and (iv) bored by this survey and

anxious to end it as quickly as possible.

0 Yes/no Follow-ups: Yes and no responses should be followed

up by the open-ended question: "Why did you vote yes/no?" 

Answers should be carefully coded to show the types of

responses, for example: (i) It is (or isn't) worth it; (ii)

Don't know; or (iii) The oil companies should pay.

0 Cross-tabulations: The survey should include a variety of

other questions that help to interpret the responses to the

primary valuation question. The final report should include

summaries of willingness to pay broken down by these

categories. Among the items that would be helpful in
58

interpreting the responses are:

Income

Prior Knowledge of the Site

Prior Interest in the Site (Visitation Rates)

Attitudes Toward the Environment

Attitudes Toward Big Business

Distance to the Site

Understanding of the Task

Belief in the Scenarios

Ability/Willingness to Perform the Task

We believe that these cross tabulations will prove useful in

interpreting and lending credibility to the responses and

possibly also in forming adjustments that can enhance

reliability.

0 Checks on Understanding and Acceptance: The above

guidelines must be satisfied without making the instrument

so complex that it poses tasks that are beyond the ability

or interest level of many participants.

Since CV interviews often present information that is new to

respondents, the questionnaire should attempt at the end to

determine the degree to which respondents accept as true the

descriptions given and assertions made prior to the valuation

question. Such an inquiry should be carried out in detail but
59

non-directively, so that respondents feel free to reject any part

of the information they were given at earlier points.

GOALS FOR VALUE ELICITATION SURVEYS

The following items are not adequately addressed by even the

best CV surveys. In the opinion of the Panel, these issues will

need to be convincingly dealt with in order to assure the

reliability of the estimates.

0 Alternative Expenditure Possibilities: Respondents must be

reminded that their willingness to pay for the environmental

program in question would reduce their expenditures for

private goods or other public goods. This reminder should

be more than perfunctory, but less than overwhelming. The

goal is to induce respondents to keep in mind other likely

expenditures, including those on other environmental goods,

when evaluating the main scenario.

Consumers can be expected to make expenditure decisions that

are adequately sensitive to other expenditure possibilities with

which they are familiar. But environmental referenda of the type

presented in CV surveys are unfamiliar and respondents may not be

aware of the large set of other expenditure possibilities that

might be offered in future CV surveys or future referenda. 

Unless informed otherwise, respondents may suppose that there is
60

only one environmental scenario that will ever be offered and

they may overspend on it.

It is not at all clear how exhaustive should be the list of

alternative public goods that are explicitly presented. If the

list is too brief, overspending can be expected. If the list is

too long, respondents will be encouraged to spread expenditures

to public goods for which there is not adequate total demand and

which therefore cannot really be offered to them. Also, if the

list gets large enough to encompass a significant fraction of

income, the gap between willingness to pay and willingness to

accept may widen.

It is also not clear what form the reminder should take. It

does not seem enough merely to list other environmental goods

since respondents would then have to guess the level of

expenditure that would be necessary to pay for the alternatives.

 The survey should probably include some statement about the

price of the alternatives, for example, the per capita

expenditure that would be required to provide the items.

0 Deflection of Transaction Value: The survey should be

designed to deflect the general "warm-glow" of giving or the

dislike of "big business" away from the specific

environmental program that is being evaluated. It is

possible that the referendum format limits the "warm glow"

effect, but until this is clear the survey design should

explicitly address this problem.
61

Economic models of consumer behavior generally are based on

the assumption that value derives from the goods and services

that are consumed, not from the process by which these goods are

allocated. But happiness that derives from charitable giving may

come mostly from the act of giving rather from the material

changes that follow from the gift. To give another example,

consumers may get pleasure from the act of shopping as well as

from ownership of the goods they purchase. Words that might be

useful to distinguish between these utility-producing events are

"consumption value" and "transaction value," the latter referring

to the process or transaction that establishes ownership.

We do not question the validity of "transaction value" or

differentiate it from "consumption value" as far as damage

assessment is concerned. But for both forms of value,

respondents need to be thinking clearly about the substitutes,

since the closer are the substitutes the less the damage that is

done. In the case of "transaction value," there are many close

substitutes to cleaning up oil spills since there are many other

charitable activities that can generate the same "warm glow" and

there are many other ways to express hostility toward big

business and modern technology.

0 Steady State or Interim Losses: It should be made apparent

that respondents can distinguish interim from steady-state
62

losses.

The quality of any natural resource varies daily and

seasonally around some "equilibrium" or "steady state" level. 

Active-use value of a resource depends on its actual state at the

time of use (and at other times), not on its equilibrium. But

passive-use value of a natural resource may derive only or mostly

from its steady state and not from its day-to-day state. If so,

full restoration at some future date eliminates or greatly

reduces passive-use loss. Surveys accordingly need to be

carefully designed to allow respondents to differentiate interim

from steady state passive-use loss.

0 Present Value Calculations of Interim Losses: It should be

demonstrated that, in revealing values, respondents are

adequately sensitive to the timing of the restoration

process.

As discussed in Section III above, the time profile of

restoration following an accident potentially is an important

determinant of active-use loss and interim passive-use loss, but

respondents may have little ability to distinguish between and to

evaluate different profiles.

0 Advance Approval: Since the design of the CV survey can

have a substantial effect on the responses, it is desirable
63

that -- if possible -- critical features be preapproved by

both sides in a legal action, with arbitration and/or

experiments used when disagreements cannot be resolved by

the parties themselves.

0 Burden of Proof: Until such time as there is a set of

reliable reference surveys, the burden of proof of

reliability must rest on the survey designers. They must

show through pretesting or other experiments that their

survey does not suffer from the problems that these

guidelines are intended to avoid. Specifically, if a CV

survey suffered from any of the following maladies, we would

judge its findings "unreliable":

-- A high nonresponse rate to the entire survey instrument

or to the valuation question.

-- Inadequate responsiveness to the scope of the

environmental insult.

-- Lack of understanding of the task by the respondents.

-- Lack of belief in the full restoration scenario.

-- "Yes" or "no" votes on the hypothetical referendum that

are not followed up or explained by making reference to
64

the cost and/or the value of the program.

0 Reliable Reference Surveys: In order to alleviate this

heavy burden of proof, we strongly urge the government to

undertake the task of creating a set of reliable reference

surveys that can be used to interpret the guidelines and

also to calibrate surveys that do not fully meet the

conditions.
65

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66

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State of Ohio v. Department of the Interior, 880 F.2d 432 (D.C.

Cir. 1989)