Rat treatment data from Mantel et al. Three rats were chosen from each of 100 litters, one of which was treated with a drug, and then all followed for tumor incidence.
rats
data(cancer, package="survival")| litter: | litter number from 1 to 100 | 
| rx: | treatment,(1=drug, 0=control) | 
| time: | time to tumor or last follow-up | 
| status: | event status, 1=tumor and 0=censored | 
| sex: | male or female | 
E. W. Lee, L. J. Wei, and D. Amato, Cox-type regression analysis for large number of small groups of correlated failure time observations, in "Survival Analysis, State of the Art", Kluwer, 1992.