10.11 Dates in R
10.11.1 Converting dates to survival time
In the melanoma example dataset, we already had the time in a convenient format for survival analysis - survival time in days since the operation. This section shows how to convert dates into “days from event”. First we will generate a dummy operation date and censoring date based on the melanoma data.
library(lubridate) first_date <- ymd("1966-01-01") # create made-up dates for operations last_date <- first_date + days(nrow(melanoma)-1) # every day from 1-Jan 1966 operation_date <- seq(from = first_date, to = last_date, by = "1 day") # create dates melanoma$operation_date <- operation_date # add sequence to melanoma dataset
Now we will create a ‘censoring’ date by adding
time from the melanoma dataset to our made up operation date.
Remember the censoring date is either when an event occurred (e.g., death) or the last known alive status of the patient.
Now consider if we only had the
operation date and
We want to create the
Surv() function expects a number (
numeric variable), rather than a
date object, so we’ll convert it: