Using contingent valuation to value a noxious weeds control program: the effects of including an unsure response category

The National Oceanic and Atmospheric Administration Contingent Valuation panel recommended that a response category be included along with the vote in favor and vote against options associated with a referendum contingent valuation question that allows individuals to express lack of a well defined opinion. However, the recommendation did not include guidance on how to analyze such responses. In this article, we describe the results of a carefully designed split sample experiment that allowed for comparison of a standard dichotomous-choice referendum treatment to a treatment that included the option of responding unsure to the contingent valuation question in addition to the standard vote in favor and vote against response categories. We are able to examine several options for dealing with the unsure responses and conclude that the unsure responses should be included in the value estimation as the respondents who choose this response category are distinct from respondents who choose the vote in favor and vote against response categories.