= Solution
Cohort A alone would give $\widehat F_A(2)=48/100=0.48$. But two-year survival is the product of first-year survival and conditional survival through the second year. Cohort B contributes a full year's information to the first factor even though it has no second-year follow-up. Omitting it discards useful early deaths and survivors. The pooled estimate $0.47$ combines the first-year information from both cohorts with the conditional second-year information from cohort A.
Thus \b[lack of two-year follow-up is handled by censoring, not by removing the entire short-follow-up cohort]. This is <cohort pooling under administrative censoring>. The criticism assumes comparable time-since-entry <survival distributions>: if enrolment cohort or calendar period genuinely changes the <hazard>, naive pooling is not justified. One should model or stratify that effect, and an estimand specifically restricted to cohort A may appropriately use its own <survival distribution>.
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