Hello all
I recently asked a selected bunch of geostatistical
'heavyweights' about the fundamental gripe Jan Merks has about the field of
geostatistics in an attempt to kill off a small seed of doubt that was growing
in my primitive geostatistical brain. For those who have pondered the
same thing, I forward Isobel Clark's response (with her
permission).
Cheers
Perry
Perry Collier
Senior Mine
Geologist
Ernest
Henry Mine
Xstrata Copper Australia
( (07) 4769 4527
4 (07) 4769 4555
* [EMAIL PROTECTED]
" http://www.xstrata.com
PO Box 527
Cloncurry QLD 4824
Australia
"Light travels faster than sound. That is why some people appear bright
until you hear them speak"
-----Original Message-----
From: Isobel Clark [mailto:[EMAIL PROTECTED]
Sent: Saturday, 11 June 2005 8:49 AM
To: Collier, Perry (Ernest Henry - Copper)
Subject: Re: JW Merks - help requested to kill off seed of doubt!
From: Isobel Clark [mailto:[EMAIL PROTECTED]
Sent: Saturday, 11 June 2005 8:49 AM
To: Collier, Perry (Ernest Henry - Copper)
Subject: Re: JW Merks - help requested to kill off seed of doubt!
Hi Perry
I remember vividly walking into the ice breaker at the 1992 Geostats
Congress in Portugal and having several people turn to me and say "what did you
DO to this guy?". I have never met Jan Merks, although I have invited him
(politely) to talk.
Merks is hung up on the seeming disparity between standard (frequentist)
statistics and geostatistics. The two main examples he gives in his seminal
article are:
(1) geostatisticians divide the variance by N when everyone knows that the
variance should be divided by N-1
(2) the kriging system is basically just a standard regression system and
hence should have effectively zero degrees of freedom.
The point he seems to have missed in (2) is quite simple: geostatistics is
a two stage process. The rules of statistical regression do not apply
because the 'covariances' (semi-variogram) calculated are based on the complete
data set. Noel Cressie says the total degrees of freedom are N(N-1) and started
to froth when I asked him this question - and he isn't a GEOstatistician but is
a top flight statistician.
In (1) his point is even more fundamental. Classical variances are divided
by N-1 because we estimate the mean from the data as well as the variance.
Semi-variograms are divided by N because we don't -- we assume the mean
difference is zero.
This is all explained at great length in Practical Geostatistics 2000,
which we sent to him 5 years ago. Of course, you do have to read it to find that
out.
Jan Merks is a brilliant man and has written some excellent stuff. However,
he is hoist by his own petard in that he also can ignore facts which conflict
with his own view of the world.
I am sure Peter will set you straight on this and wish you all good luck in
your studies.
Isobel
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