In contingent valuation surveys the category of zero bidders refers to individuals that are not willing to pay anything for the programme under analysis. Specific questions can help to identify true zero values, coming from people that are indifferent to the programme, separately from protest...

Previous empirical research has demonstrated an internal inconsistency that may occur in response strategies between the first and second valuations made to closed-ended contingent valuation questions. One possible explanation for this bound effect is the surprise of being asked the second...