Shirley McKie case – logical and statistical aspects

The Y7/Shirley McKie case – logical and statistical aspects

I am a computer programmer working in the field of statistics for industry. I suspect that the confident beliefs that Shirley McKie lied about entering the murder scene were based on an error of logic.

The case is extremely unusual because there is no reason, other than the fingerprint, to believe that any wrongdoing occurred - there is no independent evidence of a crime. This is a major and significant abnormality, the case might even be unique in the history of forensic evidence presented in a court of law. I suspect the significance of the abnormality was not taken into account when the evidence against Shirley McKie was evaluated.

Known fingerprint errors are very rare but it does not follow that we can assume that every identification will carry only a very small risk of being wrong (to do so would be a serious error of logic known as the Prosecutor’s Fallacy). After a suspect has disputed forensic evidence the likelihood of error in that case is tied (inversely) to the likelihood of the suspect’s guilt. This can only be assessed by balancing the likelihoods of the different explanations for the facts of the case.

While fingerprint errors are rare it is not rare for perpetrators of crimes to leave fingerprints while committing them. So if a fingerprint from a location connected with a crime plausibly incriminates somebody in that crime, and they deny it, this is much more likely to be the perpetrator lying than an innocent person telling the truth due to a rare error.

When a single fingerprint is the only reason to believe that wrongdoing has occurred, as well as suggesting who did it, the balance of likelihoods is not so clear. Misidentifications are rare but rare events happen. Do we have a valid reason to believe that we are not simply reacting to a rare error?

A police officer making an unauthorised entry to a secured and guarded murder scene, who would deny it if told of fingerprint evidence, would surely be a rare thing and be very unlikely if there was no reason to suspect it had happened. Committing perjury over such a thing will be even more rare and unlikely. So when evaluating the disputed fingerprint evidence in the Shirley McKie case, taking the context into account, the likelihood of one rare and unlikely explanation for the facts should be compared and balanced with the likelihood of another rare and unlikely explanation. The result of balancing two similar likelihoods is uncertainty.

If this had been recognised in 1997, suspicions that Shirley McKie was lying might have been dropped.

Steve Horn.

West Lothian

sz@hornsc.clara.co.uk

It struck me while watching news clips of Scottish Parliament inquiry into the Shirley McKie case that if, generally speaking, fingerprint identifications carry a small risk of error, then a confident belief that McKie deposited Y7 could be a case of The Texas Sharpshooter Fallacy.

Fingerprinting Innocentville is a way of looking at the logic using a few simple “back of envelope” type calculations.

When looking at expert evidence we have to be aware of Transposing the Conditional

Since 1997 the McKie case has been repeatedly studied and investigated by police officers, a procurator fiscal (prosecutor), judges, forensic experts and, in 2006, by a committee of the Scottish Parliament. In all this time nobody has thought to question the evaluation of the evidence that gave rise to the case.

LINKS:

I think that the principle described here is important and should be clearly understood by justice professionals. I have joined discussions in the CLPEX forum for fingerprint examiners and the linked pages below are copies of some of my contributions.

From a logical point of view using a denied elimination identification as a means of detecting a hidden act of wrongdoing has similarities to using mass medical screening to detect hidden diseases. In mass screening a medical test can be very accurate yet someone who tests positive will probably not have the disease.

There are logical similarities with the Sally Clark case (the Royal Statistical Society used the term “serious error of logic” in connection with that case).

More about relative likelihood and the McKie case.

An exercise to see how varying the error rate alters the likelihood of guilt produced some interesting results.