Tests of utility independence when health varies over time.

In the conventional quality adjusted life year (QALY) model, people's preferences are assumed to satisfy utility independence. When health varies over time, utility independence implies that the value attached to a health state is independent of the health state that arise before or after it. Two separate studies were conducted involving a total of 155 respondents. In study one, we conducted five tests of utility independence using a standard gamble question. Three of the tests of utility independence were repeated in study two after randomisation was introduced in order to take account of possible ordering effects. Utility independence holds in the majority of cases examined here and so our work generally supports the use of utility independence to derive more tractable models.