Some aspects need clarification: - Are start_yr and end_yr constant for a subject? - Is the combination of Subject and yr unique?   If the answer to both questions is yes, try: data want; set have(keep= Subject start_yr end_yr); by Subject; length n_start 8; retain n_start; if first.Subject then do; n_start = _n_; end; if last.Subject then do; flag = (_n_ - n_start) = end_yr - start_yr; output; end; drop n_start; run; ... View more

I don't know what do you mean by 'Robust' . Do you mean it is very slowly for big table ? You could split the big table into many small tables and write a macro to go through all these small tables. Or perhaps you want start a new session to explain it more details and let others see it too . ... View more

data test; infile datalines delimiter = ','; input solddt: date9. rxsup dose med_end_dt: date9. cluster ; format solddt yymmdd10. med_end_dt yymmdd10.; datalines; 10sep2009, 16, 200, 26sep2009, 1 02oct2009, 2, 45, 04oct2009, 1 06oct2009, 66, 90.91, 11dec2009, 1 12oct2009, 16, 200, 28oct2009, 1 21oct2009, 12, 62.5, 02nov2009, 1 03nov2009, 33, 181.82, 06dec2009, 1 12nov2009, 2, 45, 14nov2009, 1 12nov2009, 16, 200, 28nov2009, 1 04dec2009, 33, 181.82, 06jan2010, 1 14dec2009, 25, 128, 08jan2010, 1 17dec2009, 2, 45, 09dec2009, 1 15jan2010, 5, 45, 20jan2010, 2 18feb2010, 25, 90, 15mar2010, 3 ; run; The input data is provided above. Thank you. ... View more