>
>In good ol' Statview (now dearly departed) to complete a Scheffe
>test you selected the independent variables and dependent variable
>and it produced a table with the pairwise comparisons of the levels
>of the factor. I'm looking for a system that is as basic, but can be
>done using R and has documentation so I'm not guessing what I'm
>doing. I'd rather not have to do plots in R and then run over to
>dead software to do Scheffe's if possible.
>
>I checked on google and there seems to be code for a couple of
>functions out there, but I need something that has a manual.
>
>Is there a Scheffe function out there that is reasonably well
>documented, or should I consider some other method of dealing with
>this data. We have been using Scheffe for this type of analysis as I
>was under the impression it was very conservative. Tukey's HSD seems
>to be conservative as well. Should I try this? Is there a different
>approacch that is better and where can I read about it.
>
>Thanks for any help you can provide.
>
>Sam
>
It sounds like you are only interested in simple pairwise comparisons, in
which case you are better off using Tukey's HSD. This will still protect the
family-wise error rate, but will allow you to infer more differences than
you would using Scheff�'s method. Scheff�'s method is exact if you are truly
interested in all contrasts (a situation I have never come across), but it
is overly conservative when inferences are only made for pairwise
differences.
A geometric explanation of the difference between the two methods can be
found in Jason Hsu's book "Multiple Comparisons: Theory and Methods".
As you can also read in that book, there is no good reason to precede
Tukey's HSD with an ANOVA F-test. Tukey's HSD controls the family-wise error
rate anyway, so an initial F-test is superfluous.
--Jim
James A. Rogers
Manager, Nonclinical Statistics
PGR&D Groton Labs
Eastern Point Road (MS 8260-1331)
Groton, CT 06340
office: (860) 686-0786
fax: (860) 715-5445
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