Town Fingerprint Project

This imaginary exercise is to illustrate a point I have been making for some time about the Shirley McKie perjury allegation. A fingerprint investigation is normally started because a crime has taken place. In the McKie case the disputed match between her and "latent" (crime scene) print Y7 came first, then a new previously unimagined offence was suggested to explain the match. Statistical theory seems to indicate that the pre-existing crime is an essential part of the safety of fingerprinting. Using a few simple "back of envelope" type calculations I can show that this is true and demonstrate that there is no certainty at all that McKie deposited Y7.

Town Fingerprint Project

Imagine that a town decided to have all its citizens fingerprinted and the prints held in a database. The town also decided to send fingerprint teams to every house to uncover latent prints. All the latents would be checked against the database and everyone identified would have to give an explanation for why they had been at the location.

The results would show that the overwhelming majority of identifications would not be disputed because the person identified would be happy to accept that they had deposited the latent print. Only in a tiny proportion of all the identifications would the person say that they have never been at the location. There are two explanations for a disputed identification. Either the person was up to no good and is lying about it, or it was a misidentification and an innocent person is being accused of depositing the latent print.

Let's say the town has about 25,000 people and 10,000 households. In every house 400 latent prints were found. So throughout the town 4 million identifications were made (10,000 x 400). Let's assume that misidentifications run at the rate of 1 per million identifications so there will be 4 misidentifications during the project spread over the 10,000 houses. Let's also assume that crimes occurred recently in 40 houses and in 20 of these, the criminal left a latent print.

We are now going to look at the chances of finding a misidentification or someone lying at 3 different houses, using different methods to choose the house.

Case 1. A house is chosen at random.

If we pick a house by sticking a pin in a map then we are extremely unlikely to hit on one that contains a latent print where the identification is disputed. After all, most houses don’t get burgled or contain a murder, and misidentifications are very rare. The chances of finding a latent left during a crime are 0.2% (20 in 10000). The chances of stumbling across a misidentified latent are 0.04% (4 in 10000).

Case 2. A house is chosen because it contains a disputed identification.

If we choose a house because it contains a latent print where the identification was disputed then how can we weigh up the relative strengths of the two explanations (lying or misidentification)? If the person identified is to be prosecuted that is what the jury will have to do.

A good starting point is the ratio of lies to misidentifications for the town as a whole. 20 latent prints were left by criminals and there are 4 misidentifications, a ratio of 5:1. If this was put to a jury I don't think they would consider an 83% chance of guilt and 17% chance of innocence to be 'beyond reasonable doubt'.

But using the town's overall ratio is only the starting point. We need to ask if anybody saw the identified person in the house and we need to look for evidence that a crime or offence has actually taken place. If there is no evidence of a crime and nobody saw the identified person in the house then it is reasonable to say that the balance of probabilities shifts to be in favour of the 'misidentification' explanation.

Case 3. A house is chosen because it contains a crime

We choose one of the 40 houses where a recent crime happened. We now know for sure that someone has been in the house who will lie and if we are lucky he or she will have left a latent print. We have said that during 20 of the 40 crimes the criminal left a print so we have a 50% chance of finding it. The chance of finding a misidentification is the same as a house chosen at random, 0.04%. So if an identification is disputed in this house it is 1250 (50 / 0.04) times more likely to be a criminal lying than a misidentification.

Conclusion

Even with an error rate of only 1 in a million identifications, it is not safe to propose that a previously unimagined offence has occurred to explain a disputed fingerprint identification. It would only be safe to do this if the error rate could be proved to be zero. If a crime scene is chosen because a crime has occurred then fingerprinting is reasonably safe with an error rate of 1 per million identifications.

The Marion Ross murder inquiry is an example of case 3 above but the Shirley McKie perjury allegation is an example of case 2. With no other prior evidence than the fingerprint there is no certainty that Shirley McKie is lying and she should not have been prosecuted for perjury. This conclusion can be reached without looking at the quality of the fingerprint work. Instead of leading to an allegation, a disputed identification in these circumstances should be seen as a time to question whether best practice has been followed.

I believe that this exercise shows that the SCRO fingerprint experts at Shirley McKie's perjury trial unknowingly misled the jury because they did not point out that McKie's identification carries no certainty. If they misled the jury it was because they were not taught the statistical theory that underpins their profession.