[R] Question on weighted Kaplan-Meier analysis of case-cohort design

From: xiao-jun ma <gmaxma_at_gmail.com>
Date: Sun, 10 Jun 2007 13:20:09 -0700


I have a study best described as a retrospective case-cohort design: the cases were all the events in a given time span surveyed, and the controls (event-free during the follow-up period) were selected in 2:1 ratio (2 controls per case). The sampling frequency for the controls was about 0.27, so I used a weight vector consisting of 1 for cases and 1/0.27 for controls for coxph to adjust for sampling bias. Using the same weights in Kaplan-Meier analysis (survfit) gave very inaccurate survival curves (much lower event rate than expected from population). Are weighting handled differently between coxph and survfit? How should I conduct a weighted Kaplan-Meier analysis (given that survfit doesn't accept a weighted cox model) for such a design?

Any explanations or suggestions are highly appreciated,

xiaojun



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