[R] Huynh-Feldt R vs SAS Bug
Peter Dalgaard
p.dalgaard at biostat.ku.dk
Wed May 4 22:36:06 CEST 2005
Bela Bauer <bela_b at 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 at biostat.ku.dk) FAX: (+45) 35327907
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