Solved – how can I focus the log rank test in a selected period of time of follow up

kaplan-meierlogrank-testrsurvivaltime series

I am using R survdiff (survival package).
I would like to focus the analysis on the first 2 years of my survival curve (that is actually much longer, but with few cases in the long term and with possible superimposed curves between the groups).
What should I do to focus the analysis on these first 2 years only?
Should I subset the dataset?

Best Answer

Before proceeding, you should ask yourself "Why limit follow-up to two years?" If there is some rational reason for truncating follow-up at two years and you are committed to using the Kaplan-Meier method and the log-rank test, you will need to recode your censoring indicator and the follow-up time to reflect a maximum follow-up of two years:

#load required package
  install.packages("survival")
  library("survival")

#generate the data
  set.seed(42)
  time <- abs(rnorm(42, mean=0, sd=1 ))*1800
  event <- sample( c(0,0,1), 42, replace=TRUE)
  group <- sample( c(1,2), 42, replace=TRUE)

#reform data for maximum follow-up of two years
  time_730 <- ifelse(time>=730, 730, time)
  event_730 <- ifelse(time>=730, 0, event)

#logrank test at two years
  summary( Surv(time_730, event_730))

  survdiff( Surv(time_730, event_730) ~ group)

You mention overlapping curves or violation of the proportional hazards assumption. If this assumption is violated, you will need to look carefully at your curves before accepting the logrank p-value as a true test of the equality of survival experiences.

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