[R] Effect size, interactions, and main effects (Stats Question) [ffmanova]

Mark Difford mark_difford at yahoo.co.uk
Sat Jul 21 18:07:42 CEST 2007

Hi Robert,

Thank you for your reply, and for what appears to be some good/sound
practical advice on managing this kind of problem.

Best Regards,

Robert A LaBudde wrote:
> Statistical significance is "detectability", and depends upon the 
> size of the  sample as well as the effect. A large enough experiment 
> will result in statistical detectability of almost every interaction 
> action term allowed.
> This is why estimation, not testing, has become the consensus 
> recommendation in statistics.
> As a practical matter, evaluate the combined effect of your model 
> terms with and without the interaction term(s) you are worried about. 
> Is the reduction in accuracy of physical importance? If so, the 
> interaction terms are required for scientific reasons. If not, 
> present both results and indicate the acceptability (for 
> interpolation) of the simpler model.
> You should also make it your first priority to hypothecate why the 
> interaction terms are meaningful and expected. If a cause can be 
> found, it may suggest an alternate model that will eliminate 
> interactions, or satisfy your anxiety. If not, it may support your 
> argument to simplify.
> At 08:58 AM 7/21/2007, Mark wrote:
>>Dear List Members,
>>I would very much appreciate any pointers you could give me on the
>>Main Question:
>>To what extent does the "rule" that it is unreasonable to talk about main
>>effects if there are significant interactions in a model depend upon
>>size [of the significant interaction terms]?  Or is this not an issue?
>>More practically:  Suppose I were to carry out a so-called Type-II MANOVA
>>(using ffmanova) and were to find that the interaction term in a 2-way
>>analysis has borderline significance (say p = 0.045) and a small effect
>>size, whereas one of the main effects is highly signficant (say p =
>>and has a large effect size.
>>Would it in this case be reasonable for me to ignore the interaction term,
>>and talk only about main effects?  And, presuming the main question is
>>are there general guidlines concerning the relationship between level of
>>significance and effect size for interaction terms.
>>Thank you in advance for your help,
> ================================================================
> Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral at lcfltd.com
> Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
> 824 Timberlake Drive                     Tel: 757-467-0954
> Virginia Beach, VA 23464-3239            Fax: 757-467-2947
> "Vere scire est per causas scire"
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