A note on QALYs, time tradeoff, and discounting.

The theoretical model of QALYs is based on risk neutrality with respect to life years or constant proportional risk posture with respect to life years. It is shown that discounting of QALYs is inconsistent with these assumptions. For discounting to be consistent with individual preferences in a QALY model, individuals have to be risk-neutral with respect to discounted life years or exhibit constant proportional risk posture with respect to discounted life years. For the time-tradeoff method to be consistent with these assumptions, the QALY weights have to be derived by dividing the number of discounted life years in full health by the number of discounted life years in the assessed health states.