¥ Respondents may make associations among environmental goods
that the researcher had not intended. For example, if
asked for willingness to pay for improved visibility (through reduced
pollution), the respondent may actually
answer based on the health risks that he or she associates with dirty air.
¥ Some researchers argue that there is a
fundamental difference in the way that people make hypothetical decisions relative to the way they make actual decisions. For example, respondents may fail to take questions
seriously because they
will not actually be required to pay the stated amount. Responses may be unrealistically high if respondents
believe they will not have to pay for the good or service and that their answer
may influence the resulting
supply of the good.
Conversely, responses may be unrealistically low if respondents believe
they will have to pay.
¥ The payment question can either be
phrased as the conventional ÔWhat are you willing to pay (WTP) to receive this environmental asset?Õ, or in the less usual form, ÔWhat are you willing to accept (WTA) in compensation for
giving up this environmental asset?Õ In theory, the results should be very
close. However, when the two formats have been compared, WTA very significantly exceeds WTP. Critics have
claimed that this result invalidates the CVM approach, showing responses to be expressions of what individuals would
like to have happen rather than
true valuations.
¥ If people are first asked for their
willingness to pay for one part of an environmental asset (e.g. one lake in an
entire system of
lakes) and then asked to value the whole asset (e.g. the whole lake system),
the amounts stated may be similar. This is referred to as the Òembedding effect.Ó
¥ In some cases, peopleÕs expressed
willingness to pay for something has been found to depend on where it is placed
on a list of things being valued. This is
referred to as the "ordering problem."
¥ Respondents may give different
willingness to pay amounts, depending on the specific payment vehicle chosen. For example,
some payment vehicles, such as taxes, may lead to protest responses from people
who do not want increased
taxes. Others, such as a contribution or
donation, may lead people to answer in terms of how much they think their Òfair shareÓ contribution
is, rather than expressing their actual value for the good.
¥ Many early studies attempted to prompt
respondents by suggesting a starting bid and then increasing or decreasing this bid
based upon whether the respondent agreed or refused to pay a such sum. However, it has
been shown that the choice of
starting bid affects respondents final willingness to pay response.