OBJECTIVES:: This study was designed to provide the first empirical tests of the quality-adjusted life year (QALY) assumption of path independence, that is, health state utilities are invariant no matter where they appear in a sequence of health-status improvements. Specifically, we tested linear additivity of health-state utilities and diminishing marginal utility of health outcomes over time.

METHODS:: We employed a discrete-choice experiment to quantify patient treatment preferences for major depressive disorder (MDD). In a series of choice questions, participants evaluated 7 symptom-improvement sequences and out-of-pocket costs. The experiment isolated the effects of path dependence from the influence of conventional time discounting by focusing on short, 6-week treatment durations. Money-equivalent values (MEVs) were derived from deductive mixed-logit, latent-class analysis.

RESULTS: A total of 751 respondents with self-reported MDD completed the survey. The class-membership probability was 0.83 for latent-class preferences consistent with estimating relative importance weights for all symptom-improvement sequences in the study design. We found strong support for diminishing marginal utility in symptom-improvement sequences. The MEV of an initial week of normal mood was $147 (95% confidence interval: $128, $166) and a second week of normal mood was $70 ($49, $91). Furthermore, a subsequent additional week of normal mood was valued more highly when initial onset of effect occurred earlier, $147 ($128, $166) versus $29 (-$4, $64).

CONCLUSIONS: Our findings have important implications for conventional QALY calculations. QALYs treat health status and sequencing of changes in health status as independent factors. Our results suggest that health-technology assessments that evaluate treatments offering earlier health gains could be undervalued while those with slower health improvements could be overvalued. Our study highlights the importance of patients’ concerns about the timing of health improvements and is consistent with calls for a more nuanced approach to economic evaluations that are more attentive to patient perspectives.