[R] case-cohort

[Terry Therneau]

Jin Wang had an error.  My original note specified a variable that was 1 for 
subjects NOT in the subcohort, so the correct coxph call is

  coxph(Surv(edrel, rel) ~ stage + histol + age +
  	 offset(-100*(subcohort==0)) + cluster(seqno), data =ccoh.data)
  	 
This gives the same coefficients as the cch example, along with the 
infinitesimal jackknife or "robust" variance estimate.

	Terry Therneau