/* ---------------------------------------------------------
 program: PBC4-v92.SAS
 input:   pbc.sas7bdat
 does:    Time dependent covariates in Cox regression
                and test for proportionality
------------------------------------------------------------ */;

libname out 'H:\bios180\coursepack\SAS-unc-workshop-2008\';

*********************************************;
DATA A;
  SET out.PBC;
run;

***************************************************;
* Now look at whether there is strong evidence that;
* the relative risk comparing presence and absence ;
* of hepatomegaly varies over time.     *;
***************************************************;
proc phreg data=A;
  model time*delta(0)=age hepa hepat;
  hepat=hepa*time;
run;

********************************************************;
*  Questions:                                           ;
*                                                       ;
* 1) Is it OK to fit time in an interaction term when
*       there is no main effect for it included?
*       Why or why not?                       ;
*                                                       ;
* 2) What is the RR for dying from PBC comparing presence
*       and absennce of hepatomegaly according to this
*       model?  At 1000 days? At 2000 days?             ;
********************************************************;
* Now look to see if there is evidence of the non-
* proportionality of the hazards for the presence and
* absence of hepatomegaly in plots of the baseline     ;
* hazards for those two 'strata'.           ;
********************************************************;


title2 'Stratified Cox Regression for PBC data';
proc phreg data=A;
  model time*delta(0)=age;
  strata hepa;
  baseline out=B survival=s  logsurv=ls loglogs=lls;
run;

proc sgplot data=B;
step y=lls  x=time /group=hepa ;
run;