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List:       r-sig-mixed-models
Subject:    [R-sig-ME]  random factor selection
From:       Kirstin-Friederike Heise <kheise () uke ! uni-hamburg ! de>
Date:       2011-11-20 16:39:00
Message-ID: 455B451F-4896-4A94-ABCF-9488D7FAF38B () uke ! uni-hamburg ! de
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Hello R-list, 

I am trying to fit a mixed model with the library(nlme) to a continuous outcome \
variable (lnX2FTtrans => reaction times in ms, logarithmic transformation), being \
interested in the influence of factor age, as well as two separate estimates \
physiological parameters resulting from estimation of former analyses (best linear \
unbiased predictors , "blupREST", "blupTIMEST") and their interaction with age. 

After excluding non significant interactions, my model looks like this: 

mBZ2<- lme(lnX2FTtrans~age+blupREST+blupTIMEST+age*blupTIMEST , data=dta, random= \
~1|subject, na.action=na.exclude,method="REML")  

I would like to control for the influence of sex, which is not balanced among my \
(convenience) sample. I wonder wether I need to include it as fixed effect or if it \
is in any way reasonable to include it as a random factor. I read that dichotomous \
variables should rather be modeled as fixed factors? Is this a general "rule"?

Having tried to put "sex" as  random =~ sex | subject

this apparently slightly improved my model (p<.05) , due to AIC, BIC, and -2LL using \
the anova method in nlme. 

Thank you in advance for your valuable time and thoughts!

Kirstin


_____________________________________________________
Kirstin Heise
Department of Neurology,
University Medical Center Hamburg-Eppendorf
D-20246 Hamburg, Germany



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