I have a dataset for repayment loan amounts it consists of the following, perm_id( firm id), startyear endyear and date as well as repayment for each month. I am trying to accumulate all repayment amounts per id per month and roll them off as the loans expire I have tried the following code;    proc sql ; create table dsf2 as select a.perm_id,        a.date,        a.endyear,        sum(b.repayment) as srepayment     from dsf3 as a,          dsf3 as b     where a.perm_id = b.perm_id       and b.date >= a.date       and b.date < a.endyear     group by a.perm_id,        a.date,        a.endyear; quit;     run; However with the multiple overlapping loans it is giving very strange results Any ideas? I attempted to sum the amounts by date intervals however its picking up amounts that start later on and adds then to the very beginning. ... View more