When
you use a SUM statement and a BY statement with one BY variable, PROC
PRINT sums the SUM variables for each BY group that contains more
than one observation and totals them over all BY groups. (See
Summing Numeric Variables with One BY Group.)
When
you use a SUM statement and a BY statement with multiple BY variables,
PROC PRINT sums the SUM variables for each BY group that contains
more than one observation, just as it does if you use only one BY
variable. However, it provides sums only for those BY variables whose
values change when the BY group changes. (See
Summing Numeric Variables with Multiple BY Variables.)
Note: When the value of a BY variable
changes, the SAS System considers that the values of all variables
listed after it in the BY statement also change.