Re: [R] Huynh-Feldt R vs SAS Bug

From: Peter Dalgaard <p.dalgaard_at_biostat.ku.dk>
Date: Thu 05 May 2005 - 06:36:06 EST

Bela Bauer <bela_b@gmx.net> writes:

> Hi,
>
> I'm using anova.mlm sphericity tests/corrections, and I'm getting
> different values than SAS. In order to be able to use these values for
> publications, I'd need to know more about the SAS bug that is
> mentioned in the Reference Manual.
> - What exactly causes the different values?
> - Is it just a slight difference, or can I expect significant
> differences in H-F/G-G epsilons and corrected p-Values? With the data
> sets I'm using, the SAS value for H-F epsilon is almost twice the
> value from R, and I'm wondering if there's a mistake on my side or if
> it is just caused by the SAS bug.
>
> Thanks for any hints...

R has

    HF.eps <- ((n + 1) * pp * GG.eps - 2)/(pp * (n - pp * GG.eps))

where n is the degrees of freedom for the SSD matrix and pp is the dimension after transformation. As far as I could fathom from the SAS output, SAS is using

    (N * pp * GG.eps - 2)/(pp * (n - pp * GG.eps))

which coincides with the above when n == N - 1. This suggests that whoever coded up the SAS version generalised (N - 1) in the denominator to DF but not the N in the numerator.

Some fairly simple invariance considerations show that the SAS formula cannot be right - it's a bias correction for GG.eps which is calculated from the eigenvalues of the SSD, and the distribution of the SSD depends on degrees of freedom only.

>
> Bela
>
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-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)             FAX: (+45) 35327907

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Received on Thu May 05 06:44:31 2005

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