= Solution
The variance of an estimated hazard increment is large when its group has few individuals in the <risk set>. The <log-rank weights> are near zero when either $r_j^{(0)}$ or $r_j^{(1)}$ is small and are largest when both groups retain substantial information. They therefore suppress noisy late-event comparisons and weight each observed-minus-expected event by its available information. Unit weights would instead give equal influence to unstable increments from depleted risk sets.
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