Quality-Adjusted Life Years

You multiply the time spent in each health state by that state's utility value, on a scale where one is full health and zero is death, and add the products. A year in full health is one QALY; two years at a utility of 0.5 is also one QALY. When health varies continuously, the QALY is the area under the utility-time profile, the integral of utility over time. For economic evaluation, future QALYs are discounted to present value. The benefit of an intervention is the difference in QALYs between the treated and untreated paths, as Weinstein, Torrance, and McGuire describe.

From preference-based measurement of how people value health states. The two classic direct methods are the standard gamble, which finds the probability of full health (versus death) that makes a person indifferent to living in the impaired state, and the time trade-off, which asks how many years of full health are equivalent to a longer time in the impaired state. Utilities can also come from multi-attribute instruments such as the EQ-5D or Health Utilities Index, whose scoring is calibrated to population preferences. Torrance's review lays out and compares these techniques.

Both combine length and quality of life in time units, but they are built and used differently. The QALY is a measure of health gained, anchored at one for full health and zero for death, derived from individual preference-based utilities, and used in cost-utility analysis to evaluate interventions. The DALY is a measure of health lost, anchored at zero for full health and one for a state equivalent to death via disability weights, and used to quantify population burden of disease. Roughly, QALYs are something to maximize and DALYs something to minimize, so they should never be treated as interchangeable.