Hello,    I have two groups defined by whether or not they received an intervention. I want to propensity match the groups on age, sex, race, risk score and facility size. Group 1 is the treated group (n=274,678), and Group 2 is not treated (n=131,799).  I tried the following code first:   ods graphics on; proc psmatch data=inp_tbi_PS region=cs; class intervention BENE_RACE_CD GNDR_CD fac_size; psmodel intervention(Treated='1')= AGE GNDR_CD BENE_RACE_CD fac_size risk_score; match method=optimal (k=1) stat=lps caliper=0.25; assess lps var=(GNDR_CD fac_size) / weight=none plots(nodetails); output out (obs=match)=inp_tbi_PS_out lps=_Lps matchid=_MatchID; run;   I got an error message saying that "A feasible optimal fixed ratio matching that has the specified parameters does not exist".   Since the treated group is larger than the non-treated group, should I change my code to "Treated=0"?    Thanks in advance! ... View more