Scope insensitivity in contingent valuation studies of health care services: should we ask twice?

The main purpose of the present study was to test for outcome scope insensitivity. Respondents were initially asked to value one of two severe health states by way of a time-trade-off (TTO) exercise. Subsequent to the TTO exercise all respondents were asked to value an intervention, which offered a reduction in risk of falling into the health state they had evaluated. All respondents were subsequent to this initial CV exercise asked to value the same risk reduction, but in this case the outcome was death. Although our study passes the internal scope test, there is not a high degree of sensitivity to outcome. As many as 68% of respondents stated an identical maximum WTP in first and second CV valuation exercise implying that they value the interventions equally despite the fact that the health state presented in the initial CV question was deemed far better than death according to the TTO responses given by the same respondents. In contrast, the external scope test (comparison of response to initial CV across study arms) fared much better.