Dear R users,
 
Please assist me with the following problem. I have a dataset that looks like the following:
 
dat<-data.frame(
  'id'=rep(c(1,2,3),each=3),
  'time'=rep(c(1,2,3),3),
  'y'= c(2,2,NA,2,NA,NA,2,5,7)
) 

 
I wish to create a variable for dropout time in dataframe 'dat' such that the dropout time is the time to drop out by the subject as follows:
 
 
dat<-data.frame(
  'id'=rep(c(1,2,3),each=3),
  'time'=rep(c(1,2,3),3),
  'y'= c(2,2,NA,2,NA,NA,2,5,7),
  'dropout time'=c(2,2,2,1,1,1,3,3,3)
) 

Any help will be appreciated. Many thanks in advance.
 
james


      
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