Use of ELSE or SELECT.

When using IF statements, considerable processing improvements can be made by using the ELSE, ELSE IF, and / or SELECT statements. These statements are widely used and are well documented in the SAS Language manual, but it is worthwhile considering how these statements are combined before reverting to a single rule.

e.g.

IF CessReas=’E’ Then
If BusRes=’1’ Then
If District=’NI’ Then Output Dataset1;
Else Output Dataset2;
    Else If BusRes=’2’ and District=’NI’ Then Output Dataset3;
       Else Delete;
Else Delete;

is more efficient than

Else IF CessReas=’E’ and BusRes=’1’ and District =  ’NI’ Then Output Dataset2;
Else IF CessReas=’E’ and BusRes=’1’ and District ne ’NI’ Then Output Dataset2;
Else IF CessReas=’E’ and BusRes=’2’ and District=’NI’ Then Output Dataset3;
Else Delete;

because the test “CessReas” must be calculated 3 times, BusRes=1 twice, and District ne ‘NI’ an additional time more than necessary.