Evidence of good or ill health in an individual or community must be defined and quantified. Otherwise, further use of this evidence becomes meaningless. Health can be measured either directly or indirectly. Direct measurement of health in an individual implies not only the exclusion of disease, but also the assessment of biological functions (e.g. in sports or occupational medicine), or the ability to adapt to new and/or extreme stimuli, such as in aerospace medicine. Indirect measurement of health relies on the assessment of disease or death frequency in the community to which an individual belongs. This individual's health is then evaluated according to the probability of their contracting or dying of a disease, based on the frequency of disease (or deaths) in their community in terms of existing or anticipated cases. Frequencies of events in the community (cases of disease, deaths, etc.) are studied not only in absolute terms, but also in relation to the whole community (rates) or by comparison with event frequency in other groups or communities, or with the frequency of other diseases (ratios). Event rates and ratios represent some of the basic tools used indirectly to assess the health of individuals and communities.

Another indirect measure of health is based on the assessment of healthy behaviors (physical activity, frugal but balanced food intake, personal hygiene, etc.) or unhealthy habits (smoking, alcohol abuse, overeating, physical inactivity) of an individual or community representing a probability of present or future health problems related to such behaviors. 1 Many large-scale surveys such as the Canada Health Survey used this approach in the assessment of health.

The number of health indicators, indexes and scales is ever increasing. From this wide array of measurements of health, the occurrence of disease and death in terms of their rates and ratios are the most widely used tools in the assessment of population health, in etiological research and in the assessment of the effectiveness of treatment and care as well as that of prognosis. These tools are ubiquitous in research, practice and health policies. Let us have a look at them in this chapter.