If I have a data frame something like:
Value=rnorm(30)
Group = sample(c('A','B','C'), 30, replace=TRUE)
df = data.frame(Value, Group)

It seems like it should be simple to create an 'ObsID' column which indicates the observation order of each Value within each of the 3 groups.  Somehow, I can't quite see how to do it without manually sub-setting the parent data frame and then putting it back together again.  

Anyone able to get me started on a cleaner (more R-like) approach?

Thanks,

Rob

------------------------------------------
Robert W. Baer, Ph.D.
Professor of Physiology
Kirksville College of Osteopathic Medicine
A. T. Still University of Health Sciences
800 W. Jefferson St.
Kirksville, MO 63501
660-626-2322
FAX 660-626-2965

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