[R-sig-ME] random factor of type integer

Henrik Parn henrik.parn at bio.ntnu.no
Tue Jan 29 15:26:26 CET 2008

Dear all,

I have a dataset where the variables that I later will use as random 
grouping factors in a lmer-model are integers. Do I first need to 
convert them to factors?

I just tried if the example on the help page for lmer (where the 
grouping factor 'Subject' actually is a factor)...

fm1 <- lmer(Reaction ~ Days + (Days|Subject), sleepstudy)

...differed from:

Subject2 <- as.integer(as.character(sleepstudy$Subject))
fm2 <- lmer(Reaction ~ Days + (Days|Subject2), sleepstudy)

...but it did not.

Is this a general case or could the use of a random factor of integer 
type may cause problems in lmer?

Thanks in advance!


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