Question

I am trying to create a variable called strata for recurrent events. The idea is to define a variable that counts events, but filling previous records. What defines the counting is the variable event as is shown below:

id event   cov    strata      year
1   0       0        1        12
1   0       1        1        13
1   1       1        1        14
1   0       1        2        15
1   1       0        2        16
1   1       1        3        17
1   0       0        4        18
1   0       1        4        19
1   0       1        4        20

I have tried something like this at least for the event records:

id  <- c(rep(1,9), rep(2,5), rep(3,7))
event  <- c(0,0,1,0,1,1,0,0,0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 1)
cov  <- c(0,1,1,1,0,1,0,1,1, 0, 0, 0, 0, 1, 0, 1, 1, 1, 0, 1, 1)
year  <- c(seq(12,20,1), seq(12,16,1), seq(12,18,1)) 
dat  <- data.table(id,event, cov, year)

dat[,strata:=seq(.N),by="id"]

But it doesn’t work.

Any ideas?
Thanks

Answer 1

Here is an updated solution.

dat  <- data.table(id,event, cov, year)
dat[, strata := c(1L,head(cumsum(event)+1L,-1L)), by = id]