Time-trade-off (TTO) is one way of measuring utility. This is important for the calculation of quality adjusted life-years (QALYs).
In this approach, we ask individuals to trade time in one health state for a shorter period of time in a better health state. For example, ‘Would you rather live one year in a state of perfect health, or two years confined to bed?’. If we ask multiple individuals multiple questions of this type, we can analyse the pattern of their responses and calculate utilities for each health state.
For more information on utility, see quality adjusted life-years (QALYs).