The quality-adjusted life-year (QALY) is a generic measure of health-related quality of life that takes into account both the quantity and the quality of life generated by interventions. The invention and further development of the QALY was a response to the treatment of health outcomes solely in terms of survival without any weight being given to the quality of the additional years of life. A year of perfect health is scaled to be 'worth' 1 and a year of less than perfect health 'worth' less than 1. Death is commonly indicated by 0, though in some situations there may be states regarded as worse than death and which would have negative numbers attached to them. Thus, an intervention that results in a patient living for an additional five years rather than dying within one year, but where quality of life fell from 1 to 0.6 generates five years extra life with a quality of 0.6 (= 3.0) less one year of reduced quality (1 - 0.6) (= 0.4), so the (net) QALYs generated by the intervention are 3.0 - 0.4 (= 2.6).
The status of the QALY has been the subject of some debate and not a little confusion. Is it a measure of preference for health states? Is it a measure of health outcome that is independent of health states? Does it possess construct validity? Is it a utility measure of someone's preferences (the fact that its construction may entail the use of utility theory does not itself necessarily imply that it is a measure of anyone's preference)? Is it cardinal or ordinal? Is it consistent with the conventions of welfare economics or is it inescapably a part of extra-welfarism? What value judgments does it embody and what is their acceptability? What empirical forms of it exist and how do they differ?
A QALY is customarily generated using one of three empirical (ex perimental) methods: expected utility theory (in particular, the technique known as the standard gamble), the time trade-off and the person trade-off. These methods generate a form of cardinal utility measure that is either on an interval scale or a ratio scale (usually, the latter is required). The interval scale does not, however, permit interpersonal comparisons of the 'Individual A is twice as ill as individual B' sort.
The extra-welfarist interpretation of QALYs is that they are socially chosen cardinal indicators of health or health gain that are interval or ratio scales depending on their method of construction. It is as though the social welfare function is separable into different types of measure, some of which may be utility measures but one of which is, in any event, health or health gain. The outputs of public programmes, of which health is typically one, are determined directly by political decision-makers, as are the budgets allocated to each and, if each budget is efficiently used to maximize its intended outputs, the incremental cost-effectiveness of each programme is indirectly determined by the two decisions about outputs and budgets.
The weights applied to various components of the QALY may reflect preferences (for example, of people expected to be beneficiaries) but otherwise are value judgments decided in some other way by people deemed to be appropriate judges of such matters. On this interpretation, interpersonal comparisons can be explicitly made, as can (at least in principle) whole distributions of health (or health gain), thus enabling questions of equity to be addressed directly. This has given rise to various proposals for weighting QALYs according to who gets them (e.g. old vs. young, male vs. female, married vs. single), how many you already have (relatively well vs. relatively sick), and how many you have already had (a life time of chronic disability vs. a recently acquired disability).
- acronymQALY (written as quality-adjusted life-year)