[R-sig-ME] df numbers in lmer

Ben Bolker bbolker at gmail.com
Tue May 10 15:13:17 CEST 2011

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On 05/10/2011 08:14 AM, Zofia Prokop wrote:
> Hello,
> I'm wondering how to obtain df-s associated with t values for fixed factors
> in lmer. I need them in order to calculate the effect sizes (Pearson's r)
> for those fixed factors.
> One method of calculating df number I came across is to take the total
> number of observation minus the number(s) of levels of random factor(s). But
> I have my doubts, such as
> (1) what about the number of fixed factors in the model - shouldn't it be
> subtracted as well?
> (2) I can see how this method can be applied to models where random factors
> are crossed with fixed factors but what about when they are nested? Or in
> case of more comlex designs? The model I'm struggling with at the moment is
> such:
> I have 10 females and 10 males crossed in a full factorial design, resulting
> in 100 offspring trait means (one per each pairing). Fixed factors are:
> female category and male category (2 levels each). I'm fitting a model:
> siz <- lmer(sz~femcat+malecat+(1|femid)+(1|maleid))
> What about those df-s for femcat and malecat?
> I'd be very grateful for advice as well as literature suggestions (I'm very
> new to mixed modelling and trying to get my head around it).
> -Zofia

  As you may well have seen, this is a giant can of worms.  See for
example <http://glmm.wikidot.com/faq>.  I agree that if you are going to
do this by subtracting 'parameters' from the total number of
observations, you should subtract the number of fixed effect parameters
as well.

  I would guess at 100-20-3 for your df, keeping in mind that this is
going to be approximate.

  Ben Bolker

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