Shirley McKie and Sally Clark cases
The tragic Sally Clark case involved the rareness of an event being wrongly interpreted as proof that a crime has occurred. I think that the same thing happened to Shirley McKie. Paediatrician Professor Sir Roy Meadow giving evidence during Sally Clark’s trial, claimed the probability of two natural unexplained cot deaths in a family (Sudden Infant Death Syndrome) was 73 million-to-one.
The Royal Statistical Society says about this:
Aside from its
invalidity, figures such as the 1 in 73 million are very easily misinterpreted.
Some press reports at the time stated that this was the chance that the deaths
of Sally Clark's two children were accidental. This (mis-)interpretation is a
serious error of logic known as the Prosecutor's Fallacy. The jury needs to
weigh up two competing explanations for the babies' deaths: SIDS or murder. Two
deaths by SIDS or two murders are each quite unlikely, but one has apparently
happened in this case. What matters is the relative likelihood of the deaths
under each explanation, not just how unlikely they are under one explanation.
I have seen fingerprint errors described as being “very rare” or “very unlikely”, or that the rate is “infinitesimal”. Although these are not numerical, they are frequency estimates so they are only one side of the equation. Given the scale of fingerprint activity we should not be surprised if an error happens somewhere. Before we make any accusations we need to know the likelihood of the alternative explanation. In the McKie case this is that she disobeyed orders by entering a crime scene, which was well secured, without being seen, leaving no trace except a fingerprint, and is lying about it despite being confronted with a verified fingerprint identification.
When estimating the likelihood of this explanation we must disregard the fingerprint evidence. As far as I know there IS no other evidence (of any substance) so this explanation is very unlikely. No evidence means that there is no more and no less reason to believe that an unauthorised entry was made to Marion Ross’s house than any other crime scene, and McKie is no more and no less likely to have disobeyed orders than any other police officer. Do police officers do this sort of thing (including telling very risky lies about it)? More generally, what proportion of locations contain unknown acts of wrongdoing - serious enough for the perpetrator to lie about - waiting to be discovered by fingerprints? Is it even possible to assess the frequency of something which leaves no trace? A balance of similar probabilities is not enough, we must be sure that these hidden acts are very much more frequent than the misidentification estimates for it to be safe to assume that the identified person is lying, and to charge with perjury.
Contrast this with fingerprinting when used in the normal way to solve crimes. We KNOW that the alternative explanation happens frequently, very frequently – this is all the latent prints deposited by criminals while committing the crimes that are all too obvious.
(Of course, if the real rate of double SIDS is less than
the 1 in 73 million or if the SCRO methods during the Marion Ross murder
inquiry made misidentification more than just “very unlikely”, then the
likelihood of the accused being innocent becomes even higher.)
The RSS statement continues (with regard to the Prosecutor’s Fallacy):
The Court of Appeal has
recognised these dangers (R v. Deen 1993, R v. Doheny/Adams 1996) in connection
with probabilities used for DNA profile evidence, and has put in place clear
guidelines for the presentation of such evidence. The dangers extend more
widely, and there is a real possibility that without proper guidance, and
well-informed presentation, frequency estimates presented in court could be
misinterpreted by the jury in ways that are very prejudicial to defendants.
I would go further and say that nobody should be prosecuted or even assumed to be lying on the basis of a single frequency estimate (such as the assumption that fingerprint misidentifications are “very rare”). There must be good reason to believe that the other explanation is much more likely. This is a general point but surely at the very least we should know that a crime or act of wrongdoing has actually happened before someone is subjected to the ordeal of a trial or the humiliation of being assumed to be dishonest.
I think that Shirley McKie’s colleagues in the Srathclyde police made a “serious error of logic” when they assumed that her identification from latent fingerprint mark Y7 carries the same confidence as an identification which makes an incriminating connection in a crime investigation (I think that the confidence level is orders of magnitude lower). I think that Colin Boyd (then Solicitor General of Scotland) made the same error when he decided to prosecute McKie for perjury. A similar error of logic could happen again today if the principle is not understood.
RSS statement:
Sally Clark case:
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