Background: In valuation studies of the EQ-5D-5L instrument, the composite time tradeoff method (cTTO) is often used to elicit preferences. In cTTO, some health states are considered worse than dead (WTD) and are assigned negative utility values. However, these negative values correlate poorly with state severity, which suggests that cTTO is insufficiently sensitive. A recent threshold explanation has been offered to account for the lack of correlation: because the severity threshold beyond which a state is considered WTD differs between respondents, the correlation should be studied for individual respondents clustered by the number of WTD states. The results obtained in such a threshold approach were interpreted to disprove the insensitivity of the cTTO method.
Aim: To scrutinize the threshold explanation and test whether it indeed refutes the insensitivity of cTTO.
Methods: The study uses data from the EQ-5D-5L Polish valuation study, which includes cTTO responses from 1,510 participants, each of whom evaluated 10 EQ-5D-5L states. The correlation analysis and threshold approach are repeated to confirm the results from previous studies. The data are then modified in 2 contrasting ways. First, negative utilities are randomly reshuffled to test whether the threshold approach can capture cTTO insensitivity. Second, individual-level regressions are used to simulate negative values to ensure they correlate with severity at the individual respondent level, verifying whether the overall severity-utility correlation should be observed.
Results: First, reshuffling negative utilities does not change the results of the threshold approach. Hence, the threshold explanation fails to prove cTTO sensitivity. Second, when sensitivity was introduced on an individual level, a significant overall correlation between severity and negative utility arose.
Conclusion: cTTO is insensitive to severity for WTD states.
Highlights: For the composite time tradeoff method, the utility values of health states worse than dead correlate poorly with state severity, which suggests that cTTO has insufficient sensitivity.Recently, a so-called threshold explanation was offered for the lack of correlation.I show why the threshold explanation fails and why the composite time tradeoff is indeed insensitive for worse-than-dead states.
Keywords: EQ-5D-5L; composite time tradeoff; health state utility; threshold explanation; worse than dead.