Rob,
Yes, we did that. That was exactly the justification for removing
interaction terms (the interactions did not suggest troubling differences in
trends, just differences like greater and lesser slopes of trends in the
same direction). So having reviewed the interactions, I'm not worried about
opting for the main effects model in this instance. My question is more
general. The rationale for dropping consideration of the interactions was
not explicitly because the interactions did not appear to contradict the
main effects interpretation. The interactions were also perceived as minor
concerns because the magnitude of the F values associated with the
interactions were much smaller than those associated with the main effects.
It is this rationale that concerns me, given the likelihood of it being
applied in other situations if accepted in this case. In other words, the
next time interactions with smaller F's than main effects are encountered,
this reasoning could preclude bothering to look at the interactions. I'm
questioning the principle in general, not the consequences for this example.
The example just illustrates the context in which the decision was made.
-----Original Message-----
From: Rob Balshaw [mailto:Rob.Balshaw@syreon.com]
Sent: Friday, June 20, 2003 9:51 AM
To: Fowler, Mark
Subject: RE: [S] model performance with and without interactions...
Have you looked at the predictions themselves, perhaps through a series of
plots? If over the range you are likely to see in your data the
interactions do not result in substantive changes in the 'accuracty' of your
model, you might be justified in leaving them out.
Just a thought.
Rob
-- Robert Balshaw, Ph.D.
-- Senior Biostatistician, Syreon Corp.
-- Phone: 604.676.5900x220; Fax: 604.676.5911
CONFIDENTIAL
This e-mail and all attached documents are for the intended
recipient(s) only. They are confidential and may contain
privileged information; any unauthorized dissemination,
distribution, copying or printing is strictly prohibited.
If you have received this transmission in error, please
notify the sender immediately and delete this e-mail and
its content from your system.
> -----Original Message-----
> From: s-news-owner@lists.biostat.wustl.edu
> [mailto:s-news-owner@lists.biostat.wustl.edu]On Behalf Of Fowler, Mark
> Sent: Friday, June 20, 2003 3:51 AM
> To: S-news (E-mail)
> Subject: [S]
>
>
> I had presented the following ANOVA to depict the potential relevance of
> interactions, where we are seeking to determine if main effects
> alone could
> be used to represent the essentials of the model with respect to
> predictions
> of values for yland (annual time series):
>
> Df Deviance Resid. Df Resid. Dev F
> Value Pr(F)
> NULL NA NA 4519 5135.242 NA
> NA
> cfv 34 517.41791 4485 4617.824
> 18.68637 0.000000e+000
> yland 15 348.6338 4470 4269.191
> 28.539124 0.000000e+000
> mland 9 164.50611 4461 4104.684
> 22.444086 0.000000e+000
> AREA 5 105.90937 4456 3998.775
> 26.009187 0.000000e+000
> yland:mland 117 279.9389 4339 3718.836
> 2.937919 0.000000e+000
> yland:AREA 67 182.53822 4272 3536.298
> 3.345349 0.000000e+000
> mland:AREA 41 90.57283 4231 3445.725
> 2.712542 2.896125e-008
>
>
> I thought that the magnitude of the deviance associated with
> year:month and
> year:area, coupled with significance of the F test, indicated a need to
> interpret the interactions (i.e. they were potentially relevant in a
> pragmatic sense). This assertion was countered with the argument that the
> smaller F's associated with the interaction terms indicated that main
> effects would be adequate to do the job. Examination of the
> interactions was
> felt to support this argument, as predictions from the interaction model
> were mostly variations in degree, not contradictory. However I'm concerned
> that the compatability between main effects and interaction model
> predictions may be circumstantial (just happened to be true for this
> particular example), and that the interpretation of F magnitudes
> to reflect
> relevance of a term might be inappropriate. Opinions?
>
> --------------------------------------------------------------------
> This message was distributed by s-news@lists.biostat.wustl.edu. To
> unsubscribe send e-mail to s-news-request@lists.biostat.wustl.edu with
> the BODY of the message: unsubscribe s-news
|