Had the same problem recently. I used a loop. Assume your data frame is
called "data" and your units of observation are "Subject".
first.round.index=NULL
final.round.index=NULL
for(i in unique(Subject)){
first.round.index[i]=min(which(Subject==i))
final.round.index[i]=max(which(Subject==i))
}
first.round.index=first.round.index[is.na(first.round.index)==F]
final.round.index=final.round.index[is.na(final.round.index)==F]
##This gives you the row numbers for first, respectively last, observation
for each subject
##Then you can select the data rows of your data according to first- or
final.row.index, e.g.:
data.first.round=data[first.round.index, ]
data.final.round=data[final.round.index, ]
-------------------------
cuncta stricte discussurus
-------------------------
-----Ursprüngliche Nachricht-----
Von: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Im
Auftrag von Luwis Tapiwa Diya
Gesendet: Thursday, August 07, 2008 6:22 AM
An: R-help@r-project.org
Betreff: [R] Obtaining the first /or last record of a subject in
alongitudinal study
Dear R users,
I was wondering if anyone knows how to obtain(subset) the first and/or the
last record of a subject in a longitudinal setup.
Normally in SAS one uses first.variable1 and last.variable1. So my question
is that is there an R way of doing this.
Regards,
--
Luwis Diya, Phd student (Biostatistics), Biostatistical Center, School Of
Public Health, Catholic University of Leuven, U.Z. St Raphael,
Kapucijnenvoer 35, B-3000 Leuven, Belgium,
Cell: +32(0)497 22 94 83
Phone:+32(0)16 32 68 86 [Office]
Phone:+32(0)16 32 98 76 [Home]
Fax: +32(0)16 33 70 15
Email: [EMAIL PROTECTED]
http://med.kuleuven.be/biostat/staff/LD.htm
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