The WHERE statement in the PLM procedure is helpful when the item store contains BY-variable information and you want to apply the PROC PLM statements to only a subset of the BY groups.
A WHERE expression is a type of SAS expression that defines a condition. In the DATA step and in procedures that use SAS data sets as input source, the WHERE expression is used to select observations for inclusion in the DATA step or in the analysis. In the PLM procedure, which does not accept a SAS data set but rather takes an item store that was created by a qualifying SAS/STAT procedure, the WHERE statement is also used to specify conditions. The conditional selection does not apply to observations in PROC PLM, however. Instead, you use the WHERE statement in the PLM procedure to select a subset of BY groups from the item store to which to apply the PROC PLM statements.
The general syntax of the WHERE statement is
operand is something to be operated on. The operand can be the name of a BY variable in the item store, a SAS function, a constant, or a predefined name to identify columns in result tables.
operator is a symbol that requests a comparison, logical operation, or arithmetic calculation. All SAS expression operators are valid for a WHERE expression.
For more details about how to specify general WHERE expressions, see SAS Language Reference: Concepts. Notice that the FILTER statement accepts similar expressions that are specified in terms of predefined keywords. Expressions in the WHERE statement of the PLM procedure are written in terms of BY variables.
There is no limit to the number of WHERE statements in the PLM procedure. When you specify multiple WHERE statements, the statements are not cumulative. Each WHERE statement is executed separately. You can think of each selection WHERE statement as one analytic query to the item store: the WHERE statement defines the query, and the PLM procedure is the querying engine. For example, suppose that the item store contains results for the numeric BY variables A and B. The following statements define two separate queries of the item store:
WHERE a = 4; WHERE (b < 3) and (a > 4);
The PLM procedure first applies the requested analysis to all BY groups where a equals 4 (irrespective of the value of variable b). The analysis is then repeated for all BY groups where b is less than 3 and a is greater than 4.
Note that WHERE expressions in the SAS DATA step and in many procedures are specified in terms of the unformatted values of data set variables, even if a format was applied to the variable. If you specify the WHEREFORMAT option in the PROC PLM statement, the PLM procedure evaluates WHERE expressions for BY variables in terms of the formatted values. For example, assume that the following format was applied to the variable tx when the item store was created:
proc format; value bf 1 = 'Control' 2 = 'Treated'; run;
Then the following two PROC PLM runs are equivalent:
proc plm source=MyStore; show parms; where b = 2; run; proc plm source=MyStore whereformat; show parms; where b = 'Treated'; run;