Standardisation

One way to control for confounding is to stratify the analysis by the confounding variable, as we did by deriving age-specific death rates for the two countries. A special application of stratification known as standardisation Tooltip link is often used to control for the confounding effects of age so that rates of disease or mortality can be compared in populations with different age structures. Standardisation allows a single index of comparative mortality to be derived, in a way that permits comparison of mortality measures that are free of the effects of the underlying age distributions of the populations under observation.

There are two methods of standardisation, direct and indirect. On the following two pages we will consider each of these separately. The terminology of direct and indirect standardisation, common to most demography and epidemiology texts, is not particularly helpful – either in terms of understanding the distinctions between the two approaches, or in terms of implementing the procedure. We retain the terms purely for reasons of consistency.

A set of techniques used to remove as much as possible the effects of differences in age or other confounding variables when comparing two or more populations.
A technique by which the specific rates in a study population are averaged, using as weights the distribution of a specific standard population. The directly standardised rate represents what the crude rate would have been in the study population if that population had the same distribution as the standard population with respect to the variable(s) with which the adjustment or standardisation was carried out.
A population in which the age and sex composition is known precisely as result of a census or by arbitrary means. A standard population is used as a comparison group when standardising mortality rates.