This study analyzed the scope effects of respondent uncertainty in contingent valuation (CV)
by evaluating whether willingness to pay (WTP) estimates were sensitive to changes in the
magnitudes of motorized emission reductions in the city of Nairobi, Kenya. The WTP
estimates were elicited through the conventional payment card (PC), stochastic payment card
(SPC) and the polychotomous payment card (PPC) formats. While SPC and PPC formats
were used to capture respondent uncertainty, the PC format captured respondent certainty
regarding the amounts individuals were WTP for emission reductions. Based on parametric
and nonparametric analysis, the results show that certain (PC) respondents stated
significantly larger WTP amounts for larger emission reductions than for smaller reductions.
Conversely, uncertain (SPC and PPC) respondents stated smaller amounts for larger
emission reductions than certain (PC) respondents. The implication is that though
respondents were sensitive to the scope of motorized emission reductions, respondent
uncertainty lowered their sensitivity to scope.