Evaluating health related quality of life: an indifference curve interpretation for the time trade-off technique.

A commonly used method of measuring cardinal preferences for the purpose of evaluating health related quality of life is the time trade-off (TTO) technique. The TTO technique, although offered as a substitute to the standard gamble (SG) technique, is not related in a general way to any existing behavioral theory. We suggest that in the context of value function theory the TTO technique is a method which enables us to identify different points on an individual's indifference curve in his evaluation space. Creating a formal theoretical foundation for the TTO technique enables us to better understand and interpret the outcome of our measurements. We show, for example, that the current way of calculating the value preference of a given health state assumes a very specific set of specific set of value functions. This assumption has not been validated in empirical studies and thus casts doubt upon the interpretation of these values as being the value preferences of individuals for these health states.