[Spambayes] Geometric Mean calculation

[mbn]

Hi Guys,

Sorry for reviving an old topic, but I would like to get more information
about the geometric mean (GM) method devised by G. Robinson. The earliest
reference I could find is his article:

http://radio.weblogs.com/0101454/stories/2002/09/16/spamDetection.html

Where he mentions that the geometric mean is consistent with Fisher's
method, but no more details are given:

"The connection would take too much time to explain here, but it is a great
justification for why this should work really well."

I understant GM is now obsolete and has been replaced in favor of Fisher's,
but I am interested in the derivation of the method (or at least the basic
intuition behind it) and its relationship with Fisher's. I could not find
any explanation in the archives of SpamBayes. 

The most similar thing to Robinson's GM I could find is "logarithmic opinion
pool" with weights wi=1/n and constant k=1, but I have no idea whether the
two things are related. 

Any explanation about the derivation of the GM method will be greatly
appreciated.

Thanks in advance
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