Domain Analysis

It is common practice to compute statistics for subpopulations, or domains, in addition to computing statistics for the entire study population. Analysis for domains that uses the entire sample is called domain analysis (subgroup analysis, subpopulation analysis, subdomain analysis). The formation of these subpopulations of interest might be unrelated to the sample design. Therefore, the sample sizes for the subpopulations might actually be random variables.

In order to incorporate this variability into the variance estimation, you should use a DOMAIN statement. Note that using a BY statement provides completely separate analyses of the BY groups. It does not provide a statistically valid subpopulation or domain analysis, where the total number of units in the subpopulation is not known with certainty.

For more detailed information about domain analysis, see Kish (1965).