= Stratified log-rank statistic
{title2=$U=\sum_s\sum_j(O_{sj}-E_{sj})$}
Compute observed-minus-expected event contributions within each stratum's <risk set>, then add them across independent strata. Under the null, the corresponding conditional <variances> add. Standardizing gives an approximate <normal distribution> test, or its square gives a one-degree-of-freedom <chi-squared distribution> test. The comparison requires the usual exchangeability and <independent censoring> assumptions within strata.
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