You can specify a BY statement with PROC MIXED to obtain separate analyses on observations in groups defined by the BY variables. When a BY statement appears, the procedure expects the input data set to be sorted in order of the BY variables. The variables are one or more variables in the input data set.

If your input data set is not sorted in ascending order, use one of the following alternatives:

• Sort the data by using the SORT procedure with a similar BY statement.

• Specify the BY statement options NOTSORTED or DESCENDING in the BY statement for the MIXED procedure. The NOTSORTED option does not mean that the data are unsorted but rather that the data are arranged in groups (according to values of the BY variables) and that these groups are not necessarily in alphabetical or increasing numeric order.

• Create an index on the BY variables by using the DATASETS procedure (in Base SAS software).

Because sorting the data changes the order in which PROC MIXED reads observations, the sorting order for the levels of the CLASS variable might be affected if you have specified ORDER=DATA in the PROC MIXED statement. This, in turn, affects specifications in the CONTRAST or ESTIMATE statement.

For more information about the BY statement, see SAS Language Reference: Concepts. For more information about the DATASETS procedure, see the Base SAS Procedures Guide.