Historically, policies to improve population health have focused on major causes of death such as smallpox and cholera. Policy priorities have, in turn, been guided by information on mortality and life expectancy, and governments and others have worked to collect comprehensive, reliable, and valid mortality data. As death rates have decreased and life spans have lengthened, however, people have become increasingly interested in other health goals such as preventing disability, improving functioning, and relieving pain and the distress caused by other physical and emotional symptoms. With broader goals, policymakers need additional information to help them make decisions and establish priorities for public health, biomedical research, and personal health services.

For some purposes and decisions such as making individual patient care decisions or reducing postoperative infection rates, detailed clinical, behavioral, and organizational information is required. For other purposes such as understanding broad trends in the public's health or comparing the value of population health promotion strategies, it is helpful to have some overall picture or summary measure of health and well-being in addition to information on specific aspects or dimensions of health.

The development and application of summary measures of population health present complex and intriguing methodological, ethical, and political challenges. Methodologists have taken the lead in confronting these challenges, for example, in devising ways to summarize in a single measure the impact on population health of both mortality and morbidity. They generally have used one of several different methods to attach a single number—usually ranging between 0 (death) and 1 (optimal health)—to a complex of social and personal attributes that represent health status. This number has then been linked to life expectancy to form a single integrative measure of overall health. Under the overall measurement rubrics of health-adjusted life years (HALYs) or health-adjusted life expectancy (HALE), several kinds of measures have been developed. The best known include quality-adjusted life years (QALYs), years of healthy life (YHLs), and disability-adjusted life years (DALYs). Methodologists are still refining these measures to improve their reliability, validity, credibility, and ease of use.

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