[R-sig-ME] random factor selection

[Kirstin-Friederike Heise]

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




-------------- next part --------------

--
Pflichtangaben gem?? Gesetz ?ber elektronische Handelsregister und Genossenschaftsregister sowie das Unternehmensregister (EHUG):

Universit?tsklinikum Hamburg-Eppendorf; K?rperschaft des ?ffentlichen Rechts; Gerichtsstand: Hamburg

Vorstandsmitglieder: Prof. Dr. Guido Sauter (Vertreter des Vorsitzenden), Dr. Alexander Kirstein, Joachim Pr?l?, Prof. Dr. Dr. Uwe Koch-Gromus