Forensic statistics

In The Metaphysical Club, there’s a story about one of the first high visibility cases in which statistics were employed to make a point about the unlikeliness of some event happening, the Howland will forgery trial.  From Wikipedia:

In the ensuing case of Robinson v. MandellCharles Sanders Peirce testified that he had made pairwise comparisons of 42 examples of Howland’s signature, overlaying them and counting the number of downstrokes that overlapped. Each signature featured 30 downstrokes and he concluded that, on average, 6 of the 30 overlapped, 1 in 5. Benjamin Peirce showed that the number of overlapping downstrokes between two signatures also closely followed the binomial distribution, the expected distribution if each downstroke was an independent event. When the admittedly genuine signature on the first page of the contested will was compared with that on the second, all 30 downstrokes coincided, suggesting that the second signature was a tracing of the first.

Benjamin Peirce, Charles’ father, then took the stand and asserted that, given the independence of each downstroke, the probability that all 30 downstrokes should coincide in two genuine signatures was 1/(2.666 × 1021) . He went on to observe:

“So vast improbability is practically an impossibility. Such evanescent shadows of probability cannot belong to actual life. They are unimaginably less than those least things which the law cares not for. … The coincidence which has occurred here must have had its origin in an intention to produce it. It is utterly repugnant to sound reason to attribute this coincidence to any cause but design.”


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