cc: t.osborn@uea.ac.uk, anders.moberg@natgeo.su.se, hegerl@duke.edu, weber@knmi.nl, myles.allen@physics.ox.ac.uk, k.briffa@uea.ac.uk, jan.esper@wsl.ch
date: Wed, 1 Nov 2006 15:40:45 +0000
from: Martin Juckes <m.n.juckes@rl.ac.uk>
subject: Re: CPD submission
to: Eduardo Zorita <Eduardo.Zorita@gkss.de>

I've attached the document I intend to put on the MITRIE web site. Following 
Eduardo's comments, I've only put myself as author, but I'm happy to include 
anyone else who would like to endorse it.

It is important to emphasise that figure 2 of MM2005 (Energy and Environment) 
which shows a line with clearly non-zero mean and claims it is a principal 
component of centred data cannot be correct: principal components of centred 
data have zero mean. It is slightly embarassing to have missed this rather 
obvious point until now, but it is nevertheless true. Studying their code, 
and getting it to run so that I am not dependent on assuming that routines
are platform independent, allows the source of this error to be determined.

I've also attached the MM2005 paper, so you can check that their figure is
properly reproduced.

cheers,
Martin

On Wednesday 01 November 2006 14:25, Eduardo Zorita wrote:
> 
> dear co-authors,
> 
> 
> On the question of data and code -sharing, I am not sure whether Climate of 
the Past is the adequate forum, but I have
> in principle nothing against it. I see however the risk that the possible 
discussion drifts from
> the manuscript itself towards those general questions. 
> 
> Concerning the more particulat question of the errors in the code my 
MM05-ee, again I would tend to be very
> cautious. I have tried to look a little bit into the R routines that may be 
used to calculate the 
> principal components, prcomb and princomb. There are several methods to do 
it, and apparently even those R-routines do not produce the same results with 
the same data. I am not an expert in the R languange and I feel completely 
unsure to as
> what those routines do internally, e.g. whether the data are indeed centered 
or not in any internal steps.
> However, I recall that when this issue was raised by MM, Mann itself 
recognized that the calculation by MM was
> correct, i.e. the leading PC was dependent on the centering period, but that 
when choosing the correct truncation
> (i.e. keeping more PCs than just the leading one) the final results were 
insensitive to this step.
> Wegman also went through the code and apparently he found it to be ok. Of 
course, it is possible that both were wrong.
> This, together with the fact that is quite easy to overlook aspects of the 
code written by others, guards
> me against making any definitive assertions on a code written in a language 
that I do not command, the results of which I do not have the chance to test 
with my own software.  Of course, you are free to do as you think is correct, 
but please not under my undorsement.
> 
> 
> 
> 
> eduardo
> 
> 
> 
> 
> 


Attachment Converted: "c:\eudora\attach\comment_ee_figure2.pdf"

Attachment Converted: "c:\eudora\attach\mcintyre_mckitrick2005_ee.pdf"
