Scope and scale insensitivities in a contingent valuation study of risk reductions.

A contingent valuation study asking willingness to pay (WTP) for reducing the overall death risk as well as the risk for fatal and non-fatal injuries in road traffic accidents was performed in Sweden 1998. Different sub-samples were used to test for scale (different risk reductions) and scope (different outcomes) effects, existence of which implies that a respondent is capable of differentiating a WTP-answer accordingly. The results indicated that respondents needed some reference point for their valuation. For instance, dependent samples showed, contrary to independent ones, a significant difference between WTP of dying from any cause and in a traffic accident for the same relative but different absolute risk reduction. Regarding non-fatal traffic accidents, tests were performed comparing valuation of risk reductions for injuries with different outcomes but identical baseline risks and relative risk reductions. Similar to the case above, dependent samples differentiated their WTP and were willing to pay significantly more for a severe injury than for a slight one, which was not the case for independent samples.