The IRT Procedure

The GROUP statement specifies the grouping variable that defines the groups of the observations. This statement is required if you intend to do a multiple-group analysis. The values of the grouping variable can be either integers or character strings. PROC IRT analyzes the input data set and determines the number of distinct groups in the data set by counting the number of distinct values in the grouping variable. Because there is no other explicit way to specify the number of groups or the grouping values, you must make sure that all (and only) the intended groups have been indexed properly by the grouping variable in the data set for a multiple-group analysis.