Time Trade-off Method
TTO is a preference-based method for assigning utilities to health states, in which the subject is asked how much time they are willing to sacrifice from a given lifespan in one health state to have a given number of years in perfect health. Thus, if H(.) denotes the index of health status, and two styles of living are ranked H(A) > H(B), one seeks experimentally to find how much shorter the time spent with H(A) should be to be of the same utility as a longer period in state H(B). Let t be the time spent in state H(B) and x in state H(A). One then seeks experimentally to vary x until the subject is indifferent between the two states:
x.U (H(A)) = t.U(H(B))
The ratio x/t provides the utility of state H(B) relative to state H(A). If the latter is arbitrarily set at 1.0 (= perfect health), then the (ratio) scale for all such states of lower utility may be revealed. The use of TTO values in economic evaluation relies on some key assumptions, notably the acceptability of preference-based measures of health, constant proportional time trade-off and additive separability.
- acronymTTO (written as time trade-off method)