Posted by
Rich Ulrich on
Mar 18, 2011; 8:14pm
URL: http://spssx-discussion.165.s1.nabble.com/Cell-sizes-Orthoplan-tp3964555p3983179.html
If you are willing to focus on the main effects, to the
exclusion of worrying about the two-way interactions, then
your power question reduces to the two separate questions,
for 4 groups and for 5 groups. You require an N that is
the larger of what is needed to detect the two effects that
are of interest. (And, "power" is a good reason that the vast
majority of experimental designs contrast two groups, not
more.)
To examine power for more than two groups, you should also
be aware of the sort of *pattern* of difference that you
expect, or want to be able to detect. It can be easier to
detect one *big* divergent mean than to detect the difference
among a spread of results. And, certainly, easier to interpret.
I still recommend J. Cohen's book, since it discusses some
of the issues (instead of merely providing formulas).
--
Rich Ulrich
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