[R] Aggregate, max and time of max

[Afshartous, David]

All,

For data consisting of serial measurements on subjects, one may use the
aggregate function to say compute the peak response for each subject for
each design condition.  Is there a way to alter this or another one-liner to
also retain the time at which the peak occurred and thus avoid writing a
doing this via a loop?  I suppose one could attempt to employ the split
function but that's probably no simpler than employing a loop.  Sample code
below:


data = expand.grid(time = seq(1,6), subject = seq(1,20), treatment =
c("placebo", "drug"))
data.ex = cbind(y = rnorm(dim(data)[1], 5, 1), data.ex)

data.peak = aggregate(data.ex[c(1)], data.ex[c(3,4)], max)
## this provides the peak of each subject on each treatment, but time is
## lost.  Including time in the statement doesn't help clearly as then
## the peak of all the times will be calculated


David

--------------------------------------
David Afshartous, Ph.D.
Research Assistant Professor
University of Miami, Miller School of Medicine
Division of Clinical Pharmacology
1500 N.W. 12th Avenue, 15th Floor West
Miami, Florida 33136

E-mail: afshar at med.miami.edu
Phone: +1 305-243-1549