Proportional heuristics in time tradeoff and conjoint measurement.

The time-tradeoff (TTO) test is widely used to measure quality of life for different health states. Subjects are asked to equate the value of living a given period in an inferior health state to the value of living a shorter period in good health. Applications of TTOs have been criticized based on the fact that the value of future life duration is taken as the future life duration itself. The authors show that for a health state in which a subject does not want to live longer than a specified amount of time, subjects' responses do not comply with the assumption that the value of the period in inferior health is equated to the value of the shorter period in good health. Actually, preference reversals with respect to such a health state point to the use of a proportional heuristic in the TTO test. Comparisons of the TTO test in these subjects with category scaling and difference measurements also favor a proportional interpretation of the TTO test. In tests based on conjoint measurement, these subjects also appear to use a proportional heuristic. Consequences of the use of the TTO test and conjoint measurement in quality-of-life models are discussed.