Date: Tue, 20 Jul 2010 14:16:39 -0400
Reply-To: "Burleson,Joseph A." <burleson@up.uchc.edu>
Sender: "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From: "Burleson,Joseph A." <burleson@up.uchc.edu>
Subject: Re: three way interaction vs separate analysis with 2 way
interactione
In-Reply-To: <172F60DA2BE647F987BBCCD3B2E929F8@ssw.buffalo.edu>
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One can get a significant interaction in which for each gender there are opposite-sign 2-way interactions, but neither is significantly different from zero.
The 3-way interaction is still significant.
The "simple 2-way" effects are then tested, each for their own significance; they may or may not be, each in their own right, significantly different from zero. But the original 3-way interaction was explicitly NOT testing whether either (or both) was significantly different from zero. IT was testing whether the 2-way interaction was parallel for the two genders.
So: report the 3-way from the total n analysis. Then "decompose" the interaction, if desired and report the significance of each from zero.
Joe Burleson
-----Original Message-----
From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] On Behalf Of Gene Maguin
Sent: Tuesday, July 20, 2010 1:36 PM
To: SPSSX-L@LISTSERV.UGA.EDU
Subject: Re: three way interaction vs separate analysis with 2 way interactione
Ann,
>>the dichot. moderator is gender, so i just split the sample and calculated
the 2 way interactions for men and women seperately.
>>my assumption was that if i for example get a sign. interaction for men
but
not for women, that i can claim that there is a gender difference.
Gene: You can claim it but as a reviewer I don't think I'd accept it. For
instance, suppose the two-way interaction coefficient was -.03 (se=.06) for
men and +.03 (se=.07) for women. Neither is significant and the signs are
different. I wouldn't believe that women and differ.
>>but as i understand you two, you both would recommend a three way
interaction model.
>>gene you wrote the following 'These two equation sets are apples and
oranges. The answer to what to do has nothing to do with statistics. The
correct answer is determined by the question you want an answer to. '
>>can you give an example when you would go for split sample with 2 way
interactions. thanks for your helpful input!
Gene: Not offhand. But, I'd bet that if you stated your hypothesis or
question, it would imply a three way interaction.
Gene Maguin
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