to know how many 'Not a valid month' ?

[tianerhu]

proc format;
picture Monthfmt
        1-12 = '99'
		other = 'Not a valid month';
run;
proc print  data = certadv.monsal;
format month monthfmt.;
run;

how to know how many 'Not a valid month' are there in the output?

[Quentin]

proc freq data = certadv.monsal;
  tables month; 
  format month monthfmt.;
run;