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

From: Roy Sanderson <R.A.Sanderson_at_newcastle.ac.uk>
Date: Fri 29 Sep 2006 - 17:31:23 GMT

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 Sanderson
Institute for Research on Environment and Sustainability Devonshire Building
University of Newcastle
Newcastle upon Tyne
United Kingdom

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


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