Hello,    I ran a psmatch procedure to match companies by size and roa in the same industry class (Fama-French 28) and it returned some matches with a propensity score of exactly 1.  I don't know why this has happened, can you help?    My code below:    proc psmatch data=m9 region=treated; class treat ff48; psmodel treat (treated="1")= size ff48; match distance=lps method=Greedy(k=1) exact=ff48 caliper=0.5; assess lps var=(size roa)/ weight=none plots=(boxplot barchart); output out(obs=match)=OutEx4_&y matchid=_MatchID; run;   Thank you in advance, Candice   Propensity Scores propensity score matching code    ... View more