Stratification for confounding--part 2: direct and indirect standardization.

Standardization is a method used to compare observed and expected rates of a given disease/outcome by removing the influence of factors that may confound the comparison. There are two major standardization methods: one is used when the 'standard' is the structure of a population (direct method) and the other when the 'standard' is a set of specific event rates (indirect method). The direct standardization is commonly used for large populations while the indirect one is applied to populations of relatively small dimensions.