Howland Will forgery trial Guide, Meaning , Facts, Information and Description
The Howland will forgery trial was a US court case in 1868 to decide Henrietta Howland Robinson's contest of the will of Sylvia Ann Howland. It is famous for the forensic use of mathematics by Benjamin Peirce as expert witness.
| Table of contents |
|
2 Statistical analysis
2.1 Peirce's arithmetic error
3 Bibliography2.2 Testing hypotheses suggested by the data 2.3 Bayesian considerations |
Robinson v. Mandell
Sylvia Ann Howland died in 1865, leaving roughly half her fortune, of some USD 2 million, to various legatees with the residue to be held in trust for the benefit of Robinson, Howland's niece. The principal was to be distributed to various beneficiaries on Robinson's death.
Robinson produced an earlier will, leaving her the whole estate outright. To the will was attached a second and separate page, putatively seeking to invalidate any subsequent wills. Howell's executor, Thomas Mandell, rejected Robinson's claim, insisting that the second page was a forgery, and she sued.
In the ensuing case of Robinson v. Mandell, Charles Sanders Peirce testified that he had made pairwise comaprisons 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. When the, admitedly 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 the probability that all 30 downstrokes should coincide in two genuine signatures was 1 divided by 2,666,000,000,000,000,000,000. 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.
In the event, the court ruled that Robinson's testimony in support of Howland's signature was inadmissible as she was a party to the will. The statistical evidence was not called upon in judgement.
PGenuine signature α PPeirce×P(genuine signature),
- where P(genuine signature) is the probability that the signature is genuine given all the other evidence in the case. The ultimate probability of forgery is still a function of just how compelling, or otherwise, is the document's provenance.Statistical analysis
The case is one of a series of attempts to introduce mathematical reasoning into the courts. People v. Collins is a more recent example.Peirce's arithmetic error
Though Peirce gave 1/530 as 2.666...×1021, it is, in fact, 9.313...×1020.Testing hypotheses suggested by the data
Why was the particular metric of overlapping downstrokes chosen, rather than one of many other ways of quantifying the similarity of two signatures? In such situations, there is always a fear that analysts are, perhaps inadvertently, indulging in testing hypotheses suggested by the data. In this case, tiny "probabilities" can be less surprising than where the metric had originated before the data had been seen.Bayesian considerations
The argument is an example of what is often regarded as the prosecutor's fallacy. The Peirces' analysis attempts to calculate the probability that two signatures would display such a degree of similarity given that they were genuine:
However, it is often contended that what is relevant to the court is the probability that the signatures are genuine given their similarity:
To relate the two probabilities requires the use of Bayes' theorem: