On Sat, Jan 15, 2011 at 4:26 PM, Matthew Strother <rstro...@gmail.com> wrote: > I have a data set with several thousand observations across time, grouped by > subject (example format below) > > ID TIME OBS > 001 2200 23 > 001 2400 11 > 001 3200 10 > 001 4500 22 > 003 3900 45 > 003 5605 32 > 005 1800 56 > 005 1900 34 > 005 2300 23 > ... > > I would like to identify the first time for each subject, and then subtract > this value from each subsequent time. However, the number of observations > per subject varies widely (from 1 to 20), and the intervals between times > varies widely. Is there a package that can help do this, or a loop that can > be set up to evaluate ID, then calculate the values? The outcome I would > like is presented below. > ID TIME OBS > 001 0 23 > 001 200 11 > 001 1000 10 > 001 2300 22 > 003 0 45 > 003 1705 32 > 005 0 56 > 005 100 34 > 005 500 23
Since the data frame appears to be already sorted by time within ID we can do this: > transform(DF, OBS = ave(OBS, ID, FUN = function(x) x - x[1])) ID TIME OBS 1 1 2200 0 2 1 2400 -12 3 1 3200 -13 4 1 4500 -1 5 3 3900 0 6 3 5605 -13 7 5 1800 0 8 5 1900 -22 9 5 2300 -33 -- Statistics & Software Consulting GKX Group, GKX Associates Inc. tel: 1-877-GKX-GROUP email: ggrothendieck at gmail.com ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
