[R] creating baseline variable from a longitudinal sequence

[Peter Dalgaard]

john james wrote:
> Dear R-help mailing list,
> �
> Kindly help me out with this problem:
> �
> I have a dataset that is in the format below,
> ID� time� Y� Age
> 1� 0���� 195� 23.1
> 1� 2��� 204� 23.3
> 1� 4�� 202��� 23.5
> 2� 0� 170��� 22.0
> 2� 3�� 234�� 22.2
> 3� 0� 208�� 24.4
> 3� 2� 194� 24 .7
> 3� 3�� 204� 24.9
> �
> I wish to remove all the measurements at time point 0 and convert�them to a baseline variable as follows;
> �
> �ID� time� Y� Age� baseline
> 1� 2��� 204� 23.3�� 195
> 1� 4�� 202��� 23.5�� 195
> 2� 3�� 234�� 22.2��� 170
> 3� 2� 194� 24 .7���� 208
> 3� 3�� 204� 24.9���� 208
>   
Something in your mail path (or mine) is inserting funny characters in
your text, making it a bit hard to use the data for testing...

Anyways, I'd split the data frame into the baseline values and the rest
using subset() or maybe split(), rename the baseline variable(s) and
then stick things together with merge()

-- 
   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark      Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)              FAX: (+45) 35327907