[R] R: Re: Question about lme syntax

[angelo.arcadi at virgilio.it]

Dear Prof. Andrew Robinson,
I am very grateful to you for your enlightening answer

All the best

Angelo





----Messaggio originale----

Da: A.Robinson at ms.unimelb.edu.au

Data: 23-nov-2015 20.50

A: "angelo.arcadi at virgilio.it"<angelo.arcadi at virgilio.it>

Cc: "R help (r-help at r-project.org)"<r-help at r-project.org>

Ogg: Re: [R] Question about lme syntax



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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Andrew Robinson
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