Sum lagged values in a panel dataset

[dapenDaniel]

Hi,

I have a panel dataset like below.

data have;
input FirmID $ Year Count;
datalines;
1001        1992          1
1001        1993          2
1001        1994          3
1001        1995          4
1001        1996          5
1001        1997          5
1002        1992          6
1002        1993          7
1002        1994          8
1002        1995          9
1002        1996          8
1002        1997          7
;

I would like to get the sum of the count in the previous 3 years. For example, Pre3 in 1995 is 6 (1+2+3). The result I want is below.

FirmID      Year       Count       Pre3

1001        1992          1             .

1001        1993          2             .

1001        1994          3             .

1001        1995          4            6

1001        1996          5            9

1001        1997          5           12

1002        1992          6            .
1002        1993          7            .
1002        1994          8           .
1002        1995          9          21
1002        1996          8          24
1002        1997          7          25

What program do I need to use?

[Kurt_Bremser · Accepted solution]

Just add a little by-group processing to the solution for https://communities.sas.com/t5/New-SAS-User/Sum-values-in-lagged-years/m-p/623759:

data want;
set have;
by firmid;
if first.firmid
then l_count = 1;
else l_count + 1;
l_count1 = lag(count);
l_count2 = lag2(count);
l_count3 = lag3(count);
if l_count gt 3 then pre3 = l_count1 + l_count2 + l_count3;
drop l_:;
run;

[dapenDaniel]

Thank you, @Kurt_Bremser