Adjustment, or standardization, is a set of summarizing procedures that are intended to remove as far as possible the effects of differences in the composition of two or more populations in which one seeks to compare and contrast health experience, for example, by examining incidence or death rates or other health indicators in these populations. When one compares health indicators among populations, several kinds of differences in their composition can distort or confound the comparison. Common variables that influence health experience include age, sex, occupation, socioeconomic status, marital status, lifestyle factors such as cigarette smoking and sedentary occupation, urban or rural location and type of residence, and ethnic background and culture. It is possible to apply adjustment procedures for each of these and other variables in such a way as to reduce or even eliminate their influence on the apparent difference in rates among several populations.
The simplest confounding factor is a difference in age composition: One population may contain much higher proportions of older persons than another. If one compares the "crude" or adjusted death rates of these two populations, the population containing a high proportion of older people will have a higher death rate than the population containing a higher proportion of children and young adults, because older people are more likely to die than younger people.
John M. Last
(see also: Incidence and Prevalence: Rates; Rates: Age-Adjusted; Rates: Age-Specific )