It is obvious that many diseases, and their treatments, affect quality of life (or morbidity) as well as length of life (or survival, mortality). In the last two decades various methods have emerged for combining both of these outcomes in the same measure. Quality-adjusted life-years (QALYs) count the number of years of ‘full health’ or ‘perfect health’ that would be equivalent to a greater number of years in a state worse than full health. For example, 10 years lived at ‘half full health’ would yield 5 QALYs, whereas 30 years at two-thirds full health would yield 20 QALYs.

The importance of the QALY is best illustrated by the graph shown in Figure 38a. After one treatment (A: let’s say surgery) a patient lives on average for 14 years. This compares with only 11 years average survival following the medical treatment (B). On the basis of differences in expected survival, clearly treatment A would be preferred to treatment B. However, this ignores possible differences in the quality of life following treatment. In particular, while there is a slight rise in the quality of life following surgery, overall the quality of life following surgery is lower than that following medical treatment. In fact, for the last 3 or 4 years of added survival the quality of life is only marginally above zero (where zero is usually defined as ‘as bad as being dead’). As a result, patients obtain more QALYs after treatment B than treatment A.