[R] Summary statistics & plots of repeated measures data

Peter Dalgaard BSA p.dalgaard at biostat.ku.dk
Sat May 24 01:03:26 CEST 2003

Jay Pfaffman <pfaffman at relaxpc.com> writes:

> I'd like to see the effects of each of the treatments for this
> within-subject comparison.  Repeated measures ANOVA seems like the
> analysis I need.  The results of
> summary(lme(ete ~ treatment, data=allitems, random=~1 | studentkey,
>           subset=allitems$classkey==4))
> follow, but I'm not quite sure what to make of them.  In particular,
> I'm very confused about the meanings of the numbers in the Value
> column, as they bear no relation to the group means of the data in
> each of those treatments.

They're not supposed to. Not in an unbalanced design. Mostly, if there
is a substantial variation between students, the results would be
closer to that obtained from an additive linear model treating student
effects as fixed (lm(ete ~ treatment+factor(studentkey), ...)). The
treatment means is what you get if you assume zero student variation,
and in general you get some sort of average between the two.

You'll bump into those issues whatever program you choose...

   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907

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