Preferences and person trade-offs: forcing consistency or inconsistency in health-related quality of life measures?

We consider assumptions about preferences implicit in the person trade-off exercises used to derive health-related quality of life measures for the Global and US Burden of Disease Projects. Because these methods and their results have the potential of being adopted by other researchers, a critical review of this methodology and its assumptions is warranted. Exercise participants are told that quality of life valuation is approached using two different questions to reveal logical inconsistencies in each person's responses. An inconsistency is claimed to exist if a participant's two responses violate a particular mathematical relationship, and participants are forced to modify their responses to satisfy that relationship. We demonstrate that this supposed 'logical' relationship need not hold for logically consistent, rational individuals, and we prove that the relationship will in fact hold only for a particular class of social value functions exhibiting two characteristics that may not be consistent with the preferences of some participants. These results imply that the forced modification may invalidate some responses, as it may require some participants to provide final answers that are inconsistent with their true preferences. We then discuss preference characterizations in the existing person trade-off literature, from which this relationship may have been derived. Published in 2002 by John Wiley & Sons, Ltd.