Date:         Wed, 11 Jul 2001 11:33:50 -0300
Reply-To:     Dan Waschbusch <dan.waschbusch@DAL.CA>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Dan Waschbusch <dan.waschbusch@DAL.CA>
Subject:      simple interaction effects
Content-Type: text/plain; charset="iso-8859-1"

Hi,

I have a significant three way interaction (A x B x C), where each factor
is between subjects with 2 levels, and I would like to decompose it using
simple effects test through GLM procedure. That is, I'd like to be able to
test whether there is an A x B interaction at C1 and whether there is an
A x B itneraction at C2. I have been tyring to find an answer as to how to
do this, and have not yet come up with a satisfactory response. Options
I've encountered so far:

(1) Run new, seperate analyses examining A x B at each level of C. For
example,
if C = gender, then examine whether A x B is significant for males, then
do the same for females. This solution is what I've found on many internet
sites, but I'm concerned because it changes the number of subjects and
degrees of freedom, relative to the original analysis (A x B x C) that
included all factors. That's not really a simple effets test, is it?

(2) I found a "white paper" on the SPSS website that describes very
clearly teh procedure, but it's written for hte old MANOVA and doesn't
work for GLM (see http://www.spss.com/tech/stat/articles/simple.txt)
about 1/2 down, under the heading General Between Models: Estimating
Simple, Simple Main Effects, and Simple Interaction Effects")

(3) Use BMDP

(4) Use the EMEANS and COMPARE commands. I have tried this, but
it only seems to give the simple effects of one factor at every level of the
other factors (e.g. A at B1C1, A at B1C2, A at B2C1, A at B2C2), but it
doesn't seem to test interactions (i.e. A x B at C1, A x B at C2).

Can anyone tell me how to decompose a 3-way (or higher) interaction using
simple effects tests in SPSS (e.g. examine A x B at each level of C)?

Thanks,
Dan Waschbusch

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