Date:         Wed, 29 Jul 2009 10:56:10 -0700
Reply-To:     SR Millis <srmillis@yahoo.com>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         SR Millis <srmillis@yahoo.com>
Subject:      Re: Interaction
In-Reply-To:  <E08B564BAA29FE4F8EC8BAF42FE0A01F0472C19C@bubsex02.cen.csin.cz>
Content-Type: text/plain; charset=utf-8

If you include interactions, you must include the main effects as well.

SR Millis


--- On Wed, 7/29/09, Spousta Jan <JSpousta@CSAS.CZ> wrote:

> From: Spousta Jan <JSpousta@CSAS.CZ>
> Subject: Re: Interaction
> To: SPSSX-L@LISTSERV.UGA.EDU
> Date: Wednesday, July 29, 2009, 8:12 AM
> Hi Billy,
>
> In general you cannot work in this way, I think. The
> interactions-only model has significant interactions not
> because the interactions are really "significant" in itself
> in the common meaning of the word, but because they contain
> a part of the information from the main effects.
>
> Instead of the full model, try models with all main effects
> and one interaction, where you believe that the interaction
> may exist.
>
> Best regards,
>
> Jan
>
>
> -----Original Message-----
> From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU]
> On Behalf Of Billy Thompson
> Sent: Wednesday, July 29, 2009 1:59 PM
> To: SPSSX-L@LISTSERV.UGA.EDU
> Subject: Interaction
>
> Hopefully some of you stats
> ‘gurus’ can help me solve my
> problem.
>
> I have a project in which the PI is interested in the
> interaction effects
> among certain variables and not the main effects.Â
> Â  The problem results
> that when I run  the
> ‘full’ factorial model with
> all effects the interaction effects are non significant and
> the main effects are significant.  If I then remove the
> significant main effects the interaction effects then become
> significant.
>
> Is it appropriate to run the model with interaction effects
> only, thus ignoring the main effects?
>
> Thanks for your help,
>
> Billy T
>
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=====================
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