Because market prices cannot be used to value them, researchers ask a sample of people how much they are willing to pay for the public good, wildlife preservation for example.
One method of overcoming some aspects of this problem is to ask two questions (1) How much would you be willing to contribute to a specific tax fund for the whole set of items to be preserved?
Several applications of the contingent valuation method have found an absence of part-whole bias (e.g., Whitehead, Haab, and Huang, 1998).
Independent valuation, in effect valuing each project at the beginning of a sequence, will always lead to the largest of the possible willingness to pay estimates.
This result is expected for the value of public goods estimated with the contingent valuation method due to substitution and income effects (Hoehn and Randall, 1989; Carson, Flores, and Hanemann, 1998).