Friday Funny – Venal Swamp Mann

If there was justice in “the Law”, there would be a clear avenue to bring suit against Mann for his malicious deceit and name his publisher and peer reviewers as co-defendants.

Michael Mann’s non-centered principal components may have been maximizing some mixture of temperature and variance, Here’s what Ian Joliffe, Ph D and author of “Principal Component Analysis” had to say about Mann’s “Hockey Stick”:

“Apologies if this is not the correct place to make these comments. I am a complete newcomer to this largely anonymous mode of communication. I’d be grateful if my comments could be displayed wherever it is appropriate for them to appear.

It has recently come to my notice that on the following website, http://tamino.wordpress.com/2008… .. , my views have been misrepresented, and I would therefore like to correct any wrong impression that has been given.

An apology from the person who wrote the page would be nice.

In reacting to Wegman’s criticism of ‘decentred’ PCA, the author says that Wegman is ‘just plain wrong’ and goes on to say ‘You shouldn’t just take my word for it, but you *should* take the word of Ian Jolliffe, one of the world’s foremost experts on PCA, author of a seminal book on the subject. He takes an interesting look at the centering issue in this presentation.’ It is flattering to be recognised as a world expert, and I’d like to think that the final sentence is true, though only ‘toy’ examples were given. However there is a strong implication that I have endorsed ‘decentred PCA’. This is ‘just plain wrong’.

The link to the presentation fails, as I changed my affiliation 18 months ago, and the website where the talk lived was closed down. The talk, although no longer very recent – it was given at 9IMSC in 2004 – is still accessible as talk 6 at Ian Jolliffe’s recent talks

It certainly does not endorse decentred PCA. Indeed I had not understood what MBH had done until a few months ago. Furthermore, the talk is distinctly cool about anything other than the usual column-centred version of PCA. It gives situations where uncentred or doubly-centred versions might conceivably be of use, but especially for uncentred analyses, these are fairly restricted special cases. It is said that for all these different centrings ‘it’s less clear what we are optimising and how to interpret the results’.

I can’t claim to have read more than a tiny fraction of the vast amount written on the controversy surrounding decentred PCA (life is too short), but from what I’ve seen, this quote is entirely appropriate for that technique. There are an awful lot of red herrings, and a fair amount of bluster, out there in the discussion I’ve seen, but my main concern is that I don’t know how to interpret the results when such a strange centring is used? Does anyone? What are you optimising? A peculiar mixture of means and variances? An argument I’ve seen is that the standard PCA and decentred PCA are simply different ways of describing/decomposing the data, so decentring is OK. But equally, if both are OK, why be perverse and choose the technique whose results are hard to interpret? Of course, given that the data appear to be non-stationary, it’s arguable whether you should be using any type of PCA.

I am by no means a climate change denier. My strong impressive is that the evidence rests on much much more than the hockey stick. It therefore seems crazy that the MBH hockey stick has been given such prominence and that a group of influential climate scientists have doggedly defended a piece of dubious statistics. Misrepresenting the views of an independent scientist does little for their case either. It gives ammunition to those who wish to discredit climate change research more generally. It is possible that there are good reasons for decentred PCA to be the technique of choice for some types of analyses and that it has some virtues that I have so far failed to grasp, but I remain sceptical.” -Ian Jolliffe

“One, the Tiljander series – bottom mud from a Finnish lake of that name was used upside down and the disturbed top of the series, it was cautioned by the discoverer of the series, should not be used! They also discovered Mann’s truncation of the decline of more recent tree ring temperatures and its replacement by the smooth”spliced” modern temperature record. That isn’t even the full extent of the hs recipe.”

The Tiljander paper didn’t exist at the time if the original HS (MBH98), as far as I know. Mann DID use it in a much later paper, 2008 maybe. The MBH98 HS relied totally on two series of stripbark pines, grossly overweighted so as to drown out all others.

Unfortunately, many of the old links on Climate Audit and on McKitrick’s site are dead.