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Absence of Evidence

December 23, 2010

Do you remember when Donald Rumsfeld famously said, “absence of evidence is not evidence of absence.” He was trying to justify his belief that there were weapons of mass destruction in Iraq. As strange as it may seem, the maxim is an example of itself. If you go to newspaper archives you might think the maxim is from 2002. But if you do a bit of research you see that Carl Sagan, who most often gets the credit for the maxim in scientific circles, used those words in 1995 in The Demon-Haunted World. But history goes back more than that. In 1972, Richard Berendzen, the chairman of a conference of scientists (including Sagan) meeting to discuss the possibility of life elsewhere in the universe, attributed the maxim to the great British cosmologist Martin Rees. And, yes, Rees had used words to that effect a few years earlier. Actually, the English writer William Cowper (1731-1800) had written something very similar. He wrote “Absence of proof is not proof of absence.”

Why is this important? For two reasons. It demonstrates how any of us, from time to time, may not have a good handle on all the relevant facts. But more importantly, it is to stress how important this maxim is thought to be in science, in history, in law, in just about any endeavor.

imageThis is the wording of an abstract, published today, in Radiocarbon, a scientific journal of the University of Arizona. The authors are Rachel A Freer-Waters and A J Timothy Jull (pictured).

We present a photomicrographic investigation of a sample of the Shroud of Turin, split from one used in the radiocarbon dating study of 1988 at Arizona. In contrast to other reports on less-documented material, we find no evidence to contradict the idea that the sample studied was taken from the main part of the shroud, as reported by Damon et al. (1989). We also find no evidence for either coatings or dyes, and only minor contaminants.  

Now how does Jim West read this and explain it over at his blog, Zwinglius Redivivus? Like this:

The fraudulent ‘Shroud of Turin’ has been proven beyond any shadow of a doubt to be a medieval forgery.  Poor Witherington and the others who continue to assert its authenticity. What will they do now?

That is Ben Witherington, by-the-way, a prominent, highly regarded Protestant theologian. I think Ben will see through this ever-so-obvious fallacy. It is sort of like drawing a single M&M from a package, not seeing red and declaring that there are no red M&Ms.

What about John Brown’s confirmation of Ray Rogers work? Can that be ignored simply because the folks in Arizona didn’t find anything similar? What about the confirmation of Rogers’ findings provided by Bob Villarreal and several other chemist from Los Alamos? What about the robust statistical analysis published on the London School of Economics site that shows that the cloth samples are non-homogeneous? Is this to be ignored?

Remi Van Haelst, a retired industrial chemist in Belgium, noted that the results failed to meet minimum statistical standards (chi-squared tests).  He asked why the wide variance in the dates between samples. Was it because of testing errors? Or was it because the sample was not sufficiently homogeneous? Bryan Walsh, a statistician, examined Van Haelst’s analysis and further studied the measurements. He concluded that the divided samples used in multiple tests contained different levels of the C14 isotope. The overall cut sample was non-homogeneous and thus of questionable validity. Walsh found a significant relationship between the measured age of various sub-samples and their distance from the edge of the cloth. Though Walsh did not suggest invisible reweaving, it is consistent with his findings.

Giovanni Riggi, the person who actually cut the carbon 14 sample from the Shroud stated, "I was authorized to cut approximately 8 square centimetres of cloth from the Shroud…This was then reduced to about 7 cm because fibres of other origins had become mixed up with the original fabric …" Should this be ignored?

Giorgio Tessiore, who documented the sampling, wrote:  “…1 cm of the new sample had to be discarded because of the presence of different color threads.” Should this be ignored?

Edward (Teddy) Hall, head of the Oxford radiocarbon dating laboratory, had noticed fibers that looked out of place. A laboratory in Derbyshire concluded that the rogue fibers were cotton of “a fine, dark yellow strand.”  Derbyshire’s Peter South wrote: “It may have been used for repairs at some time in the past…” Can this be ignored?

Gilbert Raes, when later he examined some of the carbon 14 samples, noticed that cotton fibers were contained inside the threads. This, of course, Villarreal and his team confirmed.

Alan Adler at Western Connecticut State University found large amounts of aluminum in yarn segments from the radiocarbon sample, up to 2% by energy-dispersive x-ray analysis. Why aluminum? That was an important question because it was also possible evidence of dying. Alum is a common mordant.

Are we to draw our conclusions based on found evidence or like Donald Rumsfeld on evidence not found.

There will be more on this subject soon.

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