may wish to compute these summary numbers for any subgroup of the population, or for a population as a whole.

This, then, is the simple concept: if we have suitable measures of morbidity experience and of life expectancy, we can compute a health-weighted life-year measure—HALYs, or HALE—to summarize population health. Although this seems a simple idea, it can be complex in application. This paper presents the major methodological issues surrounding construction of such measures.

I use the terms HALYs and HALE to refer generically to summary numerical representations that are an accumulation of health-weighted life years. In the course of this overview subclasses of HALYs—e.g., quality-adjusted life years (QALYs)—are discussed as well as some differentiations within subclasses.

Brief Historical Background: Descriptive and Decision Making Roots of HALYs

The groundwork for the population-based HALY representation a was laid in the 1960s and early 1970s in publications from the U.S. Department of Health, Education, and Welfare, in the operations research literature reporting work funded by HEW. The problem was to describe the overall health of the population. For years this had been indicated by mortality rates, the decline in which began to level off some in the 1950s and 1960s. During the 1960s and 1970s researchers were looking for methods to bring into the descriptive summaries information about morbidity as well as mortality.

The earliest paper I have seen is by Sanders, who introduced the mathematical combination of a measure of functional capacity and a measure of time to make a combined measure of “effective life years.” 1 Sullivan 2 amplified on this concept as did Moriyama, 3 and later Sullivan used a stationary life table technique to compute age-specific disability-free life expectancy from census data about mortality and the National Health Interview Survey (NHIS) data about disability. 4 Sullivan multiplied the number of people in each age range’s stationary population (from the census’ life table) by the proportion of the population that was disability-free in that age range in the NHIS data. The averaged result was mathematically an early progenitor of health-adjusted life expectancy, where the health weighting was a 0 for health states involving disability and 1 otherwise. This same approach has been used more recently but with different continuous measures of overall health to compute “Years of Healthy Life” (YHL) for the U.S. population 5 and HALE for Canadians. 6

As a descriptive statistic the HALE for a population draws meaning from its broad conceptual foundations and experience we gain with it over time. A good descriptive indicator should behave about like one expects it to given the conceptual grounds it springs from, and with experience over time we will learn its quirks and associate its behavior retrospectively and prospectively with other indicators of interest. This is not unlike the fashion in which some summary economic indicators such as Gross National Product (now evolved to Gross Domestic Product), or various stock market indexes around the world have acquired meaning. b

As an input for decision making, however, a HALY measure may have more demands placed on it. In particular, one type of HALY measure called the “quality-adjusted life year” (QALY) has a history associated with welfare economics and a principle for decision making under uncertainty termed “maximizing expected utility” or the EU principle. This line of reasoning begins

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