Dear Prof. Andrew Robinson, I am very grateful to you for your enlightening answer
All the best Angelo ----Messaggio originale---- Da: a.robin...@ms.unimelb.edu.au Data: 23-nov-2015 20.50 A: "angelo.arc...@virgilio.it"<angelo.arc...@virgilio.it> Cc: "R help (r-help@r-project.org)"<r-help@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.arc...@virgilio.it <angelo.arc...@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 [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. -- 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.robin...@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/ [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
