[R] select observations from longitudinal data set

[Dimitris Rizopoulos]

one way is the following:

dat <- read.table(textConnection("id time y
1 1 10
1 2 12
1 3 15
1 6 18
2 1 8
2 3 9
2 4 11
2 5 12
3 1 8
3 4 16
4 1 9
4 5 13
5 1 7
5 2 9
5 6 11"), header = TRUE)
closeAllConnections()

val <- 4
dat. <- data.frame(id = unique(dat$id), time = val)
out <- merge(dat, dat., all = TRUE)
do.call("rbind", lapply(split(out, out$id), function (d) {
     x <- d[d$time == val, ]
     ind <- is.na(x$y)
     if (ind && any(ii <- d$time == val - 1)) {
         x$y <- d$y[ii]
     } else if (ind && any(ii <- d$time == val + 1)) {
         x$y <- d$y[ii]
     }
     x
}))


If you want the output to be a matrix (and not a data.frame), then you 
could change the do.call("rbind", lapply(...)) part with t(sapply(...)).

I hope it helps.

Best,
Dimitris


gallon li wrote:
> I have the following longitudinal data:
> 
> id time y
> 1 1 10
> 1 2 12
> 1 3 15
> 1 6 18
> 2 1 8
> 2 3 9
> 2 4 11
> 2 5 12
> 3 1 8
> 3 4 16
> 4 1 9
> 4 5 13
> 5 1 7
> 5 2 9
> 5 6 11
> ....
> 
> I want to select the observations at time 4. if the observation at time 4 is
> missing, then i want to slect the observation at time 3. if the observation
> at time 3 is also missing, then i want to select observation at time 5.
> otherwise i will put a missing value there. the selected set is like
> 
> id time y
> 1 3 15
> 2 4 11
> 3 4 16
> 4 5 13
> 5 4 NA
> ...
> 
> so the rule is (1) obs at time 4 for each id; (2) if no such obs, then look
> for obs at time 3; (3) if no such obs, then look for obs at time 5; (4)
> otherwise, NA.
> 
> 	[[alternative HTML version deleted]]
> 
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-- 
Dimitris Rizopoulos
Assistant Professor
Department of Biostatistics
Erasmus Medical Center

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