This brings us to the current debate over contingent valuation. It is generally
recognized that only stated preference methods (Mitchell and Carson 1989;
Louviere, Hensher and Swait 2000; Carson, Flores and Meade 2001) are applicable
to the estimation of passive use value. Unlike direct use of resources, where
for example, one can potentially observe individuals boating and fishing and use
these observations to build economic models permitting inference about the value
individuals place on such activities,6 passive use entails no direct involvement with
natural resources. As a result, economists are fond of saying passive use leaves no
behavioral trace.