HEALTH MEASUREMENT SCALES

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.

1. Measurement scales exist for the majority of common diagnoses, as well as for broader-ranging themes such as disability or health-related quality of life. Measures range from those that focus on a particular organ system (vision, hearing), to methods concerned with a diagnosis (anxiety or depression scales), then to scales that measure broader syndromes (emotional well-being), to measurements of overall health and, broadest of all, to measures of quality of life.
2. Scales may be generic or specific. The latter may be designed for a particular disease (such as a quality-of-life scale for cancer), but can also be specific to a particular type of person (women's health measures, patient satisfaction scales) or to an age group (child health indicators). Specific instruments are generally intended for clinical application and are designed to be sensitive to change following treatment. Generic instruments, such as the Sickness Impact Profile or the European Quality of Life Scale, permit comparisons across disease categories and are used in evaluating systems of care.
3. The purposes of measurement scales include diagnosis, prediction, and evaluation. Diagnostic scales (such as the Cambridge Mental Disorders of the Elderly Examination or CAMDEX) collect a wide variety of information from self-report and clinical ratings, and process these using algorithms that suggest differential diagnoses. Prognostic measures include Health Risk Appraisal measures (which estimate the odds that a person with certain characteristics will die from specified causes within a given time frame), or methods such as the Functional Assessment Inventory, which estimate whether a patient will be able to live independently in the community following rehabilitation. Finally, evaluative indices measure change over time and are used to indicate the impact or outcomes of care. This category forms by far the largest group of instruments, and includes both generic and disease-specific outcome measures.
4. Measurement scales may be grouped into rating scales and questionnaires; there is also the distinction between health indexes and health profiles. Cutting across these categories, there is the distinction between subjective and objective measures. Rating scales refer to methods in which an expert, typically a clinician, assesses defined aspects of health; an example is the Hamilton Rating Scale for Depression. In self-assessed measurement scales, set questions are answered by the person being rated. Both are examples of subjective measures, in which human judgment (by clinician or patient) is involved in the assessment. Objective measures involve no judgment in the collection of information (although judgment may be required in its interpretation). Subjective health measurements hold several advantages. They extend the information obtainable from morbidity statistics or physical measures by describing the quality rather than merely the quantity of function. They give insights into matters of human concern such as pain, suffering, or depression that cannot be inferred solely from physical measurements or laboratory tests, and they do not require invasive procedures or expensive laboratory analyses. Measures of either type can be summarized as a single index score, or as a profile of scores. Supporters of the profile approach argue that health or quality of life is inherently multidimensional and scores on different dimensions should be presented separately. Conversely, index scores allow ready comparisons of the impact of different medical conditions, useful, for example, in economic evaluations of health care.

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.

Ian McDowell

(see also: Assessment of Health Status; Health Risk Appraisal; Life Expectancy and Life Tables )

Bibliography

Bowling, A. (1995). Measuring Disease: A Review of Disease-specific Quality of Life Measurement Scales. Buckingham, England: Open University Press.

McDowell, I., and Newell, C. (1996). Measuring Health: A Guide to Rating Scales and Questionnaires. New York: Oxford University Press.

Spilker, B., ed. (1990). Quality of Life Assessment in Clinical Trials. New York: Raven Press.

Wilkin, D.; Hallam, L.; and Doggett, M. A. (1992). Measures of Need and Outcome for Primary Health Care. Oxford: Oxford University Press.