I'm doing some survival analysis in R using the survival package for a patient cohort. When making a Kaplan-Meier curve using survfit, it reports a median survival time. However this value is very different than simply an arithmetic median using median.
survfit(Surv(OS, vital.status) ~ 1, data = df)
reports a median ~440.
Whereas if I run
I get ~180
Even with a subset of the dataframe
sub <- subset(df, df$vital.status == 1)
survfit(Surv(OS, vital.status) ~ 1, data = sub)
I still don't get near the 440 median from the kaplan meier (I got ~ 180 again).
Why is it that this 50% median survival Kaplan meier different from the median from just a basic median. I understand that with the full dataset, the censored data is not included in the median survival. But when I subset for only events, shouldn't the arithmetic median also correspond to the half-way point? What is difference in the interpretation of both of these?