flagging the last record of a ID within a test when multiple records for an ID per test are seen.

[sandhya88]

Data have;
input id $ test$ visit result;
1 ALT 1 25
1 ALT 2 35
1 ALT 3 35
2 ALT 1 20
3 AST 1 20
3 AST 2 20;
run;

Data want;
input id$ test$ visit result flg$;
1 ALT 1 25 .
1 ALT 2 35 .
1 ALT 3 35 Y
2 ALT 1 20 Y
3 AST 1 20 .
3 AST 2 20 Y;
run;

[PaigeMiller]

data want;
    set have;
    by id;
    if last.id then flg='Y'; else flg='.';
run;

If I may be so bold, setting FLG to 'Y' or '.' is not such a good idea; setting FLG to numeric 0 or 1 is not only easier to type, but has other benefits as well (such as computing proportions that are flagged can be done by computing a mean).