The DOMAIN statement requests analysis for domains (subpopulations), in addition to analysis for the entire study population. The DOMAIN statement names the variables that identify domains, which are called domain variables.
It is common practice to compute statistics for domains. The formation of these domains might not be known at the design stage. Therefore, the sample sizes for the domains are often random. Use a DOMAIN statement to incorporate this variability into the variance estimation.
Note that a DOMAIN statement is different from a BY statement. In a BY statement, you treat the sample sizes as fixed in each subpopulation, and you perform analysis within each BY group independently.
Use the DOMAIN statement on the entire data set to perform a domain analysis. Creating a new data set from a single domain and analyzing that with PROC SURVEYPHREG yields inappropriate estimates of variance.
A domain variable can be either character or numeric. The procedure treats domain variables as categorical variables. If a variable appears by itself in a DOMAIN statement, each level of this variable determines a domain in the study population. If two or more variables are joined by asterisks (*), then every possible combination of levels of these variables determines a domain. The procedure performs a descriptive analysis within each domain that is defined by the domain variables. Domain variables must not occur in the CLASS statement.
The formatted values of the domain variables determine the categorical variable levels. Thus, you can use formats to group values into levels. For more information, see the FORMAT procedure in the Base SAS Procedures Guide and the FORMAT statement and SAS formats in the SAS Formats and Informats: Reference.