Because health is an abstract concept it cannot be measured directly using a mechanical scale as weight or length are measured. Instead, indicators of health have to be selected, and some form of numerical judgement applied to quantify or "scale" these. For example, if health is defined in terms of physical, mental, and social well-being, several indicators of each of these themes will typically be selected and a scoring system for rating a person on each indicator will be devised. Finally, a second scoring system is developed to represent the relative importance of the physical, mental, and social areas in the final rating, or health measurement scale.
The indicators included in such a scale may be recorded mechanically as in a treadmill test, or they may derive from expert judgment as in a physician's assessment of a symptom. Alternatively, they may be recorded via self-ratings, as in a patient's replies to a disability questionnaire. Most indicators of physical or mental health assess the intensity, duration, or frequency of symptoms. The application of a numerical rating scale is often quite simple (as in counting a patient's arthritic joints). Alternatively, scores may be derived from sophisticated experimental scaling protocols, such as the Time Trade-off or the Standard Gamble, which represent the severity of a disabling condition by showing how many years of life a person with that condition would be willing to lose in order to return to full health for his or her remaining years.
Because of the complexity of developing a reliable and valid health measurement, there has been a steady growth over the past half century in the range of standardized health measurement scales that are available for general use. Using the same instrument in separate studies enables direct comparisons to be drawn among them. The current repertoire of health measurements numbers in the hundreds, and these have been described in several books. These ready-made health measurement scales may be classified by (1) their topic, (2) their scope, (3) their purpose, or (4) their design.
Health measurement scales have become firmly established as a routine part of evaluating new therapies and in planning care. Newer and more sophisticated techniques for scale development are being applied to health measurement scales, and a discipline of health measurement equivalent to econometrics or psychometrics is beginning to appear. Future advances will include further consolidation of the repertoire of health measurement scales, including the replacement of some outdated methods with newer instruments. Population norms are gradually being developed that will permit fuller interpretation of scores against reference standards.