I have a question regarding survival analyses: Imagine I have a large cohort, where most participants have been given one treatment (treatment A), and a much much smaller subpopulation which has been given another treatment (say, treatment B). We follow both groups for 5 years. In group receiving treatment B, no one dies. In group A, 10% die. Now, there are obvious potential issues here around bias and sampling that could be going on. But let's say both groups are well-matched, apart say for one or two covariates. What method would be best to get some insight into whether treatment B may actually be superior, or if it's just a sampling effect or something else going on, particularly given the small sample size of group B? Thank you!