[R] Helmert contrasts for repeated measures and split-plot expts

[Roy Sanderson]

Dear R-help

I have two separate experiments, one a repeated-measures design, the other
a split-plot.  In a standard ANOVA I have usually undertaken a
multiple-comparison test on a significant factor with e.g TukeyHSD, but as
I understand it such a test is inappropriate for repeated measures or
split-plot designs.

Is it therefore sensible to use Helmert contrasts for either of these
designs?  Whilst not providing all the pairwise comparisons of TukeyHSD,
presumably the P-statistic for each Helmert contrast will indicate clearly
whether that contrast is significant and should be retained in the model.
(This seems to come with the disadvantage that the parameter values are
harder to interpret than with Treatment contrasts.)  In the
repeated-measures design the factor in question has three levels, whilst in
the split-plot design it has four.

Many thanks in advance
Roy
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Roy Sanderson
Institute for Research on Environment and Sustainability
Devonshire Building
University of Newcastle
Newcastle upon Tyne
NE1 7RU
United Kingdom

Tel: +44 191 246 4835
Fax: +44 191 246 4999

http://www.ncl.ac.uk/environment/
r.a.sanderson at newcastle.ac.uk