A note on the nature of utility in time and health and implications for cost utility analysis.

Time Trade-Off (TTO) valuations of health are widely used in economic evaluation of health care. Current approaches to eliciting TTO values, and their use in economic evaluation, rest on specific assumptions about the way utility relates to time and health. Both the assumptions themselves and evidence of violations of them are discussed in the literature - yet the issues appear not to be widely appreciated by those using and applying TTO in economic evaluation. This paper adds to that literature by demonstrating both the requirements of TTO and violations of these assumptions in terms of the underlying indifference curve maps and utility functions. The advantage of this approach is that it demonstrates very clearly a number of fundamental problems for the way TTO values are currently elicited and used in cost utility analysis. In essence, it is extremely unwise to assume that the current 'tariffs' of TTO values, such as those widely used in cost utility analysis to inform health sector decisions in many countries can be applied irrespective of the duration of the health states to which they are assigned. The estimates of QALYs that result will, quite often, simply be wrong. We conclude by pointing to a number of possible solutions.