# Interpreting Median Survival vs Median in survival data

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.

e.g.

survfit(Surv(OS, vital.status) ~ 1, data = df)

reports a median ~440.

Whereas if I run

median(df$OS) I get ~180 Even with a subset of the dataframe sub <- subset(df, df$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?

Thanks!

• I think you need to look again at your sentence starting 'I understand that' which is where you are going astray I think. – mdewey Oct 24 '17 at 16:27
• Do you mind elaborating? – Jonathan Huang Oct 24 '17 at 16:34