In order to calculate quality adjusted life years (QALYs) from time trade-off (TTO) responses, individual preferences are required to satisfy the constant proportional time trade-off (CPTTO) assumption. Respondents who use a simple proportional heuristic may appear to satisfy CPTTO but will in fact generate preference reversals for states that are associated with a maximal endurable time (MET). Using data from 91 respondents, the study reported here examines the extent to which valuations satisfy the CPTTO assumption and the extent to which they might be generated by the proportional heuristic. The results suggest that respondents are using a proportional heuristic that casts doubt on the validity of using the TTO method to calculate QALYs for health states that are associated with MET preferences
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