DALY Versus QALY: Two Ways to Measure Health Loss

Disability-adjusted life years (DALYs) and quality-adjusted life years (QALYs) both squeeze the length and the quality of a life into one number, which is why they are so easy to confuse. They are built for different jobs and they run in opposite directions. A DALY measures health that is lost: on the weighting scale behind it, 0 means full health and 1 means death, and one DALY equals one year of healthy life lost. A QALY measures health that is held or gained: on its scale, 1 means full health and 0 means death, and one QALY equals one year in full health. Reading a study correctly begins with knowing which metric it uses and which way its scale points.

The DALY: counting what disease takes away

The DALY is the currency of the World Health Organization's Global Burden of Disease work. As WHO defines it, one DALY can be thought of as one lost year of healthy life, and the DALYs for a condition are the sum of two parts: years of life lost to premature death (YLL) and years lived with disability (YLD).

The YLL side is close to intuition. It multiplies the number of deaths by a standard life expectancy at the age each death occurred, so a death at 30 generates more lost years than a death at 80. The YLD side is where the metric earns its name. It takes how many people live with a condition, how long they live with it, and a disability weight that scores how much that condition reduces health.

Those disability weights are the conceptual heart of the DALY, and their scale is the source of most confusion. A disability weight runs from 0, meaning a year lived in full health, to 1, meaning a health state judged equivalent to death. A mild, well-controlled condition carries a small weight; a severe, disabling state carries a large one. Because a DALY is a bad thing being counted, the arithmetic is designed so that more disease means more DALYs. Public health uses the metric to compare the toll of very different problems on a common footing, from road injuries to depression to diabetes, and to see where the largest share of a population's lost health sits.

The QALY: counting what care can give back

The QALY comes from health economics rather than epidemiology, and it flips the scale. Here the quality weight, often called a utility, runs from 1 for full health down to 0 for death. Multiply the utility of a health state by the time spent in it and you get QALYs. A year at a utility of 0.75 yields 0.75 QALYs; two years at that level yield 1.5.

Utilities are usually drawn from standardized instruments that describe a health state across several dimensions, such as the EQ-5D, and then convert the description into a single number using values elicited from the general public. Because the QALY is a good thing being counted, interventions are judged by how many QALYs they add. That is why the QALY is the standard outcome in cost-effectiveness analysis. Divide the extra cost of a treatment by the extra QALYs it produces and you get a cost per QALY, a figure that lets a health system compare a cancer drug against a hip replacement against a screening program in the same units. Agencies that appraise value, such as the United Kingdom's NICE, work from an estimated cost-per-QALY threshold; one analysis of NHS spending produced a central estimate of roughly £10,000 per QALY, below the figures often quoted in policy debate. Educational, not medical advice.

Why the two are not interchangeable

The most reliable way to keep them straight is to remember the direction of the scale. QALYs are something you want to maximize; DALYs are something you want to minimize. A weight of 1 is the best possible outcome in a QALY analysis and the worst possible outcome on a disability weight. Skim a table without checking which convention is in force and you can read a result exactly backwards.

The differences go beyond direction. In a careful comparison of the two calculations published in Health Policy and Planning, Franco Sassi showed that quality-of-life weights and disability weights are derived differently and are not the same numbers with the signs reversed. QALY utilities are typically preference-based values elicited from population samples, while DALY disability weights have historically come from structured valuation exercises. The methods have also differed in features such as age weighting, which classic DALY calculations applied and QALYs generally did not, and in the technical detail of how future years are discounted. Because of these differences, converting one to the other is an approximation, and Sassi's worked examples show the two can diverge for the same intervention.

They also answer different questions. A DALY estimate describes a burden: how much health a population is losing to a disease, and where the largest gaps are. A QALY estimate feeds a decision: whether the health gained from a specific intervention justifies its cost. Burden does not automatically imply value. A condition can carry an enormous DALY burden while offering few cost-effective options to reduce it, and a modest-burden condition can be worth investing in because an intervention delivers QALYs cheaply.

Reading burden and value studies with both in mind

When a report says a condition causes a certain number of DALYs, it is making a statement about lost health across a population, weighted by both early death and time lived in reduced health. When a report says an intervention costs a certain amount per QALY, it is making a statement about value for money relative to other uses of the same budget. Neither number is a verdict on how much a person's life is worth; both are accounting tools that make otherwise incomparable health outcomes comparable, each with its own assumptions baked in.

The practical habit worth keeping is simple. Identify the metric, confirm the direction of its scale, ask where the weights came from, and check whether the study is describing a burden or appraising a decision. Do that, and the two most common health-measurement numbers in policy stop being interchangeable jargon and start telling you two genuinely different things.