Hi Angelo,
it's dangerous to fit a model that includes interaction effects but omits
main effects. Among other things, what can happen is that the statistical
tests become scale dependent, which is most unattractive.
I think that you should include the main effects in your model, even as
nuisance variables, and test the interaction using the model that includes
them.
BTW, your question might better be located with the mixed-effects models
special interest group.
https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
Best wishes
Andrew
On Mon, Nov 23, 2015 at 9:19 PM, angelo.arcadi at virgilio.it <
angelo.arcadi at virgilio.it> wrote:
> Dear list,
> I need an help to understand the syntax of lme to fit my model according
> to the analysis I want to perform.
>
> My dependent variable resulted from a perceptual experiment in which
> responses of participants were measured twice for each provided stimulus.
> My goal is to verify whether the responses depend on two properties of the
> participants that I know to be related to each other (height and weight, so
> they need to be considered together as an interaction). More importantly, I
> need to understand how this relationship modulates according to the type of
> stimulus participants were presented to.
>
> Based on my understanding of lme syntax, the formula I have to use should
> be the following (because I am only interested in the interaction factor of
> Weight and Height)
>
> lme_dv <- lme(dv ~ Weight:Height:Stimulus_Type, data = scrd, random = ~
1
> | Subject)
>
> Am I correct?
>
>
> Thank you in advance
>
> Best regards
>
> Angelo
>
>
>
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>
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--
Andrew Robinson
Deputy Director, CEBRA, School of Biosciences
Reader & Associate Professor in Applied Statistics Tel: (+61) 0403 138 955
School of Mathematics and Statistics Fax: +61-3-8344
4599
University of Melbourne, VIC 3010 Australia
Email: a.robinson at ms.unimelb.edu.au
Website: http://www.ms.unimelb.edu.au/~andrewpr
MSME: http://www.crcpress.com/product/isbn/9781439858028
FAwR: http://www.ms.unimelb.edu.au/~andrewpr/FAwR/
SPuR: http://www.ms.unimelb.edu.au/spuRs/
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