Quantifying quality of life for economic analysis: time out for time tradeoff.

The "Time trade-off" (TTO), is the most widely used method to "quality adjust" life years for "QALYs" in cost utility analysis. In this paper we ask if it is theoretically likely that the TTO is valid for this use. The TTO consists in a trade off between longevity and quality of life. Firstly, we argue that it is impossible to control for all factors that may influence one's willingness to sacrifice lifetime. Secondly, that longevity and quality of life are too closely interrelated for the hypothetical trade off to reveal real preferences. Thirdly, that the TTO handles the value of a life year inconsistently because it simultaneously assumes that it changes (as an outcome measure) and that it doesn't change (as a currency unit). Lastly, we ask whether the difficulties stem from an inherent contradiction in trying to quantify quality of life. The problems theoretical validity an internal consistency, contrast the use of the results as exact measurements. We conclude that cost utility analysis based on TTO cannot be trusted as a tool for setting priorities in health.