BASE= and COMPARE= specify the data sets to compare. OUTSTATS= creates the output data set DIFFSTAT. NOPRINT suppresses the procedure output. The ID statement specifies IDNUM as the ID variable. PROC COMPARE uses the values of IDNUM to match observations. 
proc compare base=emp95_byidnum compare=emp96_byidnum
             outstats=diffstat noprint;
   id idnum;
run;