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Purpose: The QALY model is today the primary method for incorporating patient preferences into medical decision and cost-effectiveness analyses. In the QALY model, quality of health is given weight proportional to health duration. Typical patient goals addressed by QALY models, such as eliminating pain or increasing mobility, are time-modulated in the sense that their importance decreases with life duration and disappears entirely if life duration is zero. Other possible patient goals, such as an author wanting to complete a book or a parent wanting a child graduate from college, have importance that is not time-modulated. Such goals, which we call extrinsic goals, retain their importance even if life duration is zero, and cannot be adequately captured by the QALY model. Here we explore extensions of the QALY model that can capture both time-modulated and extrinsic goals.
Methods and results: We consider von Neuman-Morganstern utility functions that account for life duration, time-modulated goals, and extrinsic goals. We extend the preference assumptions underlying the QALY model, such as the zero condition and standard gamble independence (Miyamoto et al 1998), to this more general setting, and show that under such assumptions, the extrinsic goal component of utility combines additively (rather than multiplicatively) with quality-adjusted life duration.
We examine duration surrogates for extrinsic goals, situations in which an extrinsic goal is adequately represented by the desire to survive for a particular duration. We show that two previously reported empirical violations of the QALY model – maximum endurable time preference, and the unwillingness to trade off life duration for quality when duration is short – can be accounted for when the utility function includes a duration surrogate for an extrinsic goal.
Finally, we revisit a published analysis of the decision to undergo carotid endarterectomy and show that the inclusion of a duration surrogate for an extrinsic goal can alter optimal treatment choice due to short-term mortality risks.
Conclusions: Von Neuman-Morganstern utility functions that include an extrinsic goal component can not only account for observed violations of the QALY model, but can do so prescriptively, thereby providing a coherent basis for including such goals in decision and cost-effectiveness analyses.
See more of Opening Plenary Session (6)
See more of The 26th Annual Meeting of the Society for Medical Decision Making (October 17-20, 2004)