Carson et al. 1994. reported a study using an explicit list design, which they
described as a demonstration of sensitivity to scope. Unfortunately, a basic flaw of
their study invalidates their conclusions. The study was concerned with the valuation
of the damage that deposits of DDT in the soil of LA Harbor has caused to the reproductive ability of two salient species of birds Peregrine Falcon, American
Bald Eagle. and two relatively obscure species of fish White Croaker, Kelp Bass..
The authors observed higher SWTP when the description of the problem involved
all four species than when it involved only the fish. Of course, the results are
equally consistent with the hypothesis that WTP to restore two important species
of endangered birds is higher than WTP to restore two relatively obscure species of
fish. The hypothesis of judgment by maximum suggests that the value attached to
the four species would not be much higher than the value attached to the most
important of these species. The results of an informal replication of the LA Harbor
study, using ratings of importance, were generally consistent with this hypothesis
Levav, 1996.. There is no reason to expect that the results of CV using explicit lists
will satisfy the add-up test