[R] getting means by group within time point for data on multiple lines (long rather than wide file)

[John Sorkin]

I have a long (rather than wide file), i.e. the data for each subject is on multiple lines rather than one line. Each line has the following layout:
subject group time value
I have two groups, multiple subjects, each subject can be seen up to three times a time 0, and at most once at times 4 and 8.
An example of the data follows:

1 control 0 100
1 control 0 NA
1 control 0 55
1 control 4 100
1 control 8 100

2 exp 0 99
2 exp 0 67
2 exp 0 66
2 exp 4 110
2 exp 8 200

I need to get means by group (control vs. exp) within time (0,4,8). The means should include only those subjects who have at least one observation at each time point (0, 4, 8). I also need to determine the number of subjects who contribute data at each time-point by group. Any suggestion on how to get them means would be appreciated. Sad to say I worked on this for four hours last night without coming to any understanding how this can be done. UGG!  

Thank you,
John




> John David Sorkin M.D., Ph.D.
> Professor of Medicine
> Chief, Biostatistics and Informatics
> University of Maryland School of Medicine Division of Gerontology and Geriatric Medicine
> Baltimore VA Medical Center
> 10 North Greene Street
> GRECC (BT/18/GR)
> Baltimore, MD 21201-1524
> (Phone) 410-605-7119
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