Group Processing Using the BY Statement

Like the CLASS statement, the BY statement specifies variables to use for categorizing observations.

Syntax, BY statement:

BY variable(s);

variable(s) specifies category variables for group processing.

But BY and CLASS differ in two key ways:

Unlike CLASS processing, BY-group processing requires that your data already be sorted or indexed in the order of the BY variables. Unless data set observations are already sorted, you must run the SORT procedure before using PROC MEANS with any BY group.

CAUTION:

If you do not specify an output data set by using the OUT= option, PROC SORT overwrites the initial data set with newly sorted observations.
The layout of BY-group results differs from the layout of CLASS group results. Note that the BY statement in the program below creates four small tables; a CLASS statement would produce a single large table.
```
proc sort data=clinic.heart out=work.heartsort; 
   by survive sex; 
run; 
proc means data=work.heartsort maxdec=1; 
   var arterial heart cardiac urinary; 
   by survive sex; 
run;
```
Figure 9.8 BY Groups Created by PROC MEANS

Tip
The CLASS statement is easier to use than the BY statement because it does not require a sorting step. However, BY-group processing can be more efficient when your categories might contain many levels.

Last updated: January 10, 2018

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