Re: Covariate in ANCOVA question

On Oct 24, 8:57 pm, David Winsemius <doe_s...@xxxxxxxxxxx> wrote:
Bruce Weaver <bwea...@xxxxxxxxxxxx> wrote innews:1193226596.924562.47100@xxxxxxxxxxxxxxxxxxxxxxxxxxx:

--- snip ---

So because there are 3 groups we should not think about type I erors in
post-hoc testing? Why, then, do the texts I have talk about controlling for
alpha and use three group examples?

--
David Winsemius


Good question. I can only speculate about the reasons. Fisher's LSD
fell out of favour generally because it does not control the family-
wise alpha well when there are 4 or more groups. I think that over
time, we (collectively) have forgotten that it *does* control family-
wise alpha when there are 3 groups. Howell's book is the only one I'm
aware of that says this.

Re textbook examples, I imagine that authors are just trying to keep
things as simple as possible (e.g., reducing the number of
calculations) when demonstrating the various multiple comparison
procedures.

--
Bruce Weaver
bweaver@xxxxxxxxxxxx
www.angelfire.com/wv/bwhomedir
"When all else fails, RTFM."

.

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