OPTIONS LS=75 PS=40;

DATA a;

 INFILE 'strata.dat' ;

 INPUT case status time tx stratum;
 
PROC sort; by stratum;

PROC PHREG;

MODEL time*status(0) = tx;

BY stratum;

PROC PHREG;

MODEL time*status(0) = tx;

STRATA stratum;

RUN;