comparison of alternative certainty calibration techniques in contingent valuation

A field test of two types of certainty calibration techniques in contingent valuation of public lands indicated that a 10-point certainty scale reduced WTP estimates by about half. Adjusting for uncertainty via a 'Not Sure' option did not reduce WTP estimates but the variance increased. There are several differences between these two ways of accounting for respondents' uncertainty, which may suggest why they provide different WTP value estimates and variances.