[R] Obtaining the first /or last record of a subject in a longitudinal study

[Chuck Cleland]

On 8/7/2008 6:21 AM, Luwis Tapiwa Diya wrote:
> 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,

   How about a combination of aggregate() and head() or tail()?

 > library(nlme)

 > aggregate(Oxboys[,-1], list(Subject = Oxboys$Subject), head, n=1)
    Subject age height Occasion
1       10  -1  126.2        1
2       26  -1  132.2        1
3       25  -1  135.5        1
4        9  -1  132.7        1
5        2  -1  136.9        1
6        6  -1  142.4        1
7        7  -1  141.3        1
8       17  -1  134.9        1
9       16  -1  142.8        1
10      15  -1  137.5        1
11       8  -1  141.7        1
12      20  -1  146.5        1
13       1  -1  140.5        1
14      18  -1  145.5        1
15       5  -1  145.8        1
16      23  -1  144.5        1
17      11  -1  142.5        1
18      21  -1  143.9        1
19       3  -1  150.0        1
20      24  -1  147.8        1
21      22  -1  147.4        1
22      12  -1  149.9        1
23      13  -1  148.9        1
24      14  -1  151.6        1
25      19  -1  156.9        1
26       4  -1  155.7        1

 > aggregate(Oxboys[,-1], list(Subject = Oxboys$Subject), tail, n=1)

..

-- 
Chuck Cleland, Ph.D.
NDRI, Inc. (www.ndri.org)
71 West 23rd Street, 8th floor
New York, NY 10010
tel: (212) 845-4495 (Tu, Th)
tel: (732) 512-0171 (M, W, F)
fax: (917) 438-0894