= Solution
At each event time within a stratum, let $r_A,r_B$ be the group <risk sets> and let $d$ be the total number of observed failures. Under the null of equal <hazard functions>, the expected A count is $d r_A/(r_A+r_B)$. The <log-rank statistic> adds observed minus expected A counts over event times; the <stratified log-rank statistic> first makes these comparisons within each patient stratum, then adds across patients. Let $V$ be the sum of the corresponding null conditional variances. The standardized statistic $U/\sqrt V$ has an approximate standard <normal distribution> under the null, and its square has an approximate one-degree-of-freedom <chi-squared distribution>. Compare it with the appropriate quantile for a two-sided test, under the usual <independent censoring> and exchangeability assumptions.
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