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
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:
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