Key points are not available for this paper at this time.
Gregory L. Poe, Michael P. Welsh, Patricia A. Champ, Measuring the Difference in Mean Willingness to Pay When Dichotomous Choice Contingent Valuation Responses Are Not Independent, Land Economics, Vol. 73, No. 2 (May, 1997), pp. 255-267
Poe et al. (Thu,) studied this question.