The most common unit of measurement of health benefit in this equation is the quality adjusted life year, or QALY (66). QALYs offer a way to integrate changes in both length and quality of life produced by an intervention; they are calculated as the average number of additional years of life gained from the intervention, multiplied by a utility judgment of the quality of life in each of those years. The QALY concept does not indicate how these weights are to be derived, but this is typically done using a scaling task such as the standard gamble, described later in this chapter. After the utility for a given state has been estimated, QALYs are calculated by multiplying the number of years to be expected in each state by the utility for that state. For example, if a statement of “Choice of work or performance at work very severely limited; person is moderately distressed by this” were rated 0.942, and if a person remained in this state for 10 years, the QALY would be 9.42 years (66). Torrance and Feeny gave the example of a person placed on antihypertensive pharmacotherapy for 30 years, which prolongs his life by 10 years at a quality level of 0.9. The need for continued drug therapy reduces his quality of life by 0.03. Hence, the QALYs gained would be 10 × 0.09 − 30 × 0.03 = 8.1 years (50). Many refinements have been proposed to this basic approach. For example, QALYs may be adjusted to reflect the individual’s preference or aversion for risktaking. This characterizes the “healthy years equivalent” (HYE) indicator, which permits the rate of trade-off between length and quality of life to depend on the expected remaining life span (65).