Future prospects - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 341

Professor Edward J. Wegman, to perform an independent examination of the
hockey stick controversy. His result is available at Wegman et al. (2006).
According to Wegman et al. (2006):

''The controversy of Mann's methods lies in that the proxies are centered on
the mean of the period 1902-1995, rather than on the whole time period ...

Principal component methods are normally structured so that each of the proxy
data series are centered on their respective means and appropriately scaled .In

the MBH approach the authors make a simple seemingly innocuous and some-
what obscure calibration assumption. Because the instrumental temperature
records are only available for a limited window, they use instrumental tempera-
ture data from 1902-1995 to calibrate the proxy data set. This would seem
reasonable except for the fact that temperatures were rising during this period,
so that centering on this period has the effect of making the mean value for any
proxy series exhibiting the same increasing trend to be de-centered low. Because
the proxy series exhibiting the rising trend are de-centered, their calculated
variance will be larger than their normal variance when calculated based on
centered data, and hence they will tend to be selected preferentially as the first
principal component. Thus, in effect, any proxy series that exhibits a rising trend
in the calibration period will be preferentially added to the first principal
component.''

...

Wegman et al. (2006) went on to say:

''The centering of the proxy series is a critical factor in using principal
components methodology properly. It is not clear that the MBH Team even
realized that their methodology was faulty at the time of writing the MBH paper.
The net effect of the de-centering is to preferentially choose the so-called hockey
stick shapes. While this error would have been less critical had the paper been
overlooked like many academic papers, the fact that their paper fit some policy
agendas has greatly enhanced their paper's visibility. Specifically, global warming
and its potentially negative consequences have been central concerns of both
governments and individuals. The hockey stick reconstruction of the temperature
graphic dramatically illustrated the global warming issue and was adopted by the
IPCC and many governments as the poster graphic. The graphic's prominence
together with the fact that it is based on incorrect use of PCA puts Dr. Mann and
his co-authors in a dicult face-saving position.''

The findings of Wegman et al. (2006) are quite lengthy and only a very brief
summary is given here:

1. In general, they found the papers by MBH somewhat obscure and incomplete.

(This writer found the same.)

2. In general, they found the criticisms by McIntyre and McKitrick to be valid and

their arguments to be compelling.