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The random card sort method and respondent certainty in contingent valuation: an exploratory investigation of range bias.

Willingness to pay (WTP) values derived from contingent valuation surveys are prone to a number of biases. Range bias occurs when the range of money values presented to respondents in a payment card affects their stated WTP values. This paper reports the results of an exploratory study whose aim was to investigate whether the effects of range bias can be reduced through the use of an alternative to the standard payment card method, namely, a random card sort method. The results suggest that the random card sort method is prone to range bias but that this bias may be mitigated by restricting the analysis to the WTP values of those respondents who indicate they are 'definitely sure' they would pay their stated WTP.