use nickel, clear
stset dataout, f(tumnas) origin(dataass) enter(datain) id(id)
stdes
*Full Cohort results
stcox afe_10 yfe_15 yfe2_15 logexp, nohr nolog

stcascoh, a(0.3) seed(123)
*Prentice method follows
stcox afe_10 yfe_15 yfe2_15 logexp, nohr robust nolog

*Self and Prentice method follows
stcox afe_10 yfe_15 yfe2_15 logexp, offset(_wSelPre) nohr robust nolog 

*Barlow method follows
stcox afe_10 yfe_15 yfe2_15 logexp, offset(_wBarlow) nohr robust nolog

*Proportional hazards model with Self-Prentice variance estimate
stselpre afe_10 yfe_15 yfe2_15 logexp, nohr