Almost like Vincent. data controls ;      input age 2. something $4.;      datalines; 23 aded 23 aaaa 23 add 23 agb 23 aef 23 add 23 agb 23 aef 23 add 23 agb 23 aef 23 add 23 agb 23 aef 24 aa 24 tt 24 ll 25 aa 25 rr 25 hh ; run; data cases ;      input age 2. somethingelse $2.;      datalines; 23 A 24 B run; data matches; merge cases (in=a) controls (in=b); by age; if age ne lag(age) then n=0; n+1; if a and b and n le 10 then output; run; Ksharp ... View more

Astounding is a good one .Have already given you a detail explanation . I just want give an example to demonstrate it . For each of data step iteration , I set ina=0 and inb=0. when this obs belong to A or B, ina or inb will be set 1 respectively . A        id       1   1 2 2 2 B id 1 1 1 2 2 A           B      ina   inb 1            1       1      1 1            1       1      1 .             1       0      1 2             2       1       1 2            2        1       1 2            .          1       0 If you didn't set ina=0 and inb=0, then both of them will always be 1 until reaching the next BY group. Ksharp ... View more