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Monday, August 7, 2006

The purpose of the Detail is to help keep you informed of the current state of affairs in the latent print community, to provide an avenue to circulate original fingerprint-related articles, and to announce important events as they happen in our field.
Breaking NEWz you can UzE...
compiled by Jon Stimac

33 Years Later, Escapee is Captured UNION DEMOCRAT, CA - Aug 3, 2006 ...fingerprint check revealed man was wanted for a 1973 escape...

Trademark Gloves Lead Police to Thief KATHIMERINI, GREECE - Aug 4, 2006 ...pair of surgeon's gloves was allegedly the trademark of an arrested suspect...

Hot Prints Jump-start Cold Case  CHICAGO SUN-TIMES, IL - Aug 1, 2006 ...21-year-old slaying solved by matching a fingerprint from a lamp to a print of the suspect...

Jailers Say New Fingerprint Law May Be Hardship KENTUCKY.COM - Jul 31, 2006 ...jailers to collect fingerprints of every inmate booked into the jail...

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No major updates on the site this week.


Last week

Steve Everist brought us the third installment of his Latent Adobe Photoshop Series - working with the Channel Mixer within Adjustment Layers.

This week

Stephen Horn relates the Shirley McKie case to statistics.


Statistical Foundation of the Shirley McKie Perjury Allegation
By Stephen Horn,
Industrial Statistician

I wish to draw attention to an unusual aspect of the Shirley McKie perjury allegation that makes it very different from a normal criminal case. The match between McKie and latent print Y7 was found during the ‘elimination’ phase of the fingerprint work in a criminal inquiry (the phase where crime scene fingerprint marks are eliminated from the inquiry because they match with people who have a legitimate reason to be at the location). The accusation of perjury came AFTER the match was found and is dependent on the match being disputed. My understanding of statistical theory is that these factors greatly increase the chances of the case being based on an erroneous fingerprint match (misidentification) compared with a normal investigation.

Why are we talking about THIS fingerprint match?

In a normal case based on fingerprint evidence, the police decide that a particular fingerprint identification is key to their case as a result of their theory of events. Whether the accused person disputes the match or accepts it as a correct identification may affect how the defence is conducted, but it does not play a part in the prosecution’s selection of the match.

The fingerprint match between Y7 and Shirley McKie was the subject of the perjury trial BECAUSE it was disputed. If it had been a correct identification and Shirley McKie had agreed that it was her print, then Y7 would just have been ‘eliminated’ in the normal way and that would have been the end of the matter. This filtering out of the vast majority of routine undisputed matches creates a self-selecting sample. This sample (the disputed fingerprint matches found during the elimination phase) is likely to have a very different probability of error from the whole population (all fingerprint matches). I wonder if the prosecuting authorities considered this when they decided to prosecute Shirley McKie for perjury.

Prosecutor’s Fallacy

Normally the first thing we know is that a crime has been committed, then fingerprint work is used to help solve it. In the Shirley McKie case, we do not even know that a crime (perjury) has been committed. The only reason anyone suspects that Shirley McKie committed perjury is because fingerprint misidentifications are very rare.

But rare events DO happen. The Prosecutor’s Fallacy is to state that the probability of innocence is the same as the probability of the rare event happening to any one specified person or event.

In an area such as Europe in every period of, say, ten years there must be billions of elimination fingerprint comparisons carried out. If the probability of a misidentification is in the order of tens of millions to one then we can assume that there will be misidentifications running in the order of 10s to 1000s every decade. Usually these will go unnoticed because the people being tested at this point have every right to be at the location. What was unusual in the McKie case were the events that brought this fingerprint match to the attention of the prosecutor.

Note that a normal criminal case does not fall foul of the Prosecutor’s Fallacy because it has a limited number of latent (crime scene) fingerprint marks that can lead to suspects being identified. This puts a boundary on the number of fingerprint comparisons that will be carried out as part of the investigation and puts an upper limit on the opportunities for misidentification.

Importance of ‘probability’

When a fingerprint expert says that a crime scene mark was left by a particular person, he or she is saying that the probability that the wrong person has been identified is so small that it puts the matter beyond reasonable doubt. Probability is what makes us trust fingerprint evidence enough to convict an accused person without supporting evidence; so prosecuting authorities and fingerprint experts have the onerous responsibility of making sure that the probability of error is so low that guilt is beyond reasonable doubt.

For every fingerprint comparison (checking one mark at a crime scene against one person’s fingerprint) there will be a very small chance that the person will be wrongly identified. This may be due to human error - bad workmanship by fingerprint experts - or perhaps even with the best fingerprint work the wrong person can be identified by chance (I understand that this has never been proved). It may not be possible to calculate a numerical value for this probability, but it must still exist.

Probability of error in any one criminal case is the probability of error in one fingerprint comparison multiplied by the number of comparisons in the inquiry. If you have 2 comparisons then there are two opportunities to find the wrong person. 1,000 comparisons and it is 1,000 times more likely that the case is based on the wrong person.

What makes this case so different from a normal criminal case is the almost unlimited quantity of fingerprint comparisons that could have led to a Shirley McKie type case (when considering probability we have to talk about A Shirley McKie case, not THE Shirley McKie case). All it takes is for a fingerprint match to be disputed and for this to be stated under oath in a trial.

Probability varies after the event

Probability is not fixed at the time of an event such as a fingerprint match being found or a lottery win.

If I approach a National Lottery player at random, I can be very very sure that they have not won the Jackpot. If they tell me otherwise, I can confidently accuse them of lying. This is because the odds of winning the lottery are 1 in 14 million and I have chosen a player AT RANDOM. If I am sitting next to someone on a train who tells me that they have won the lottery I would have no reason to doubt them because they came to my attention by a non-random process. If I gatecrash a lottery winners’ party, the probability of choosing a lottery winner from among the guests is very high. So we have three different probabilities all based around the same lottery win.

The match between fingerprint Y7 and Shirley McKie was brought to the attention of the prosecutor, and then to our attention, by a non-random process. She had reason to be near the location of Y7 but denies leaving the print, and then maintains this all the way to a perjury trial - this is a selecting process which will result in the match having a higher probability of being based on an error than a fingerprint match chosen at random.

NOTE: I think that the two points “Probability varies after the event” and “Prosecutor’s Fallacy” are actually the same logical point expressed in two different ways. The “Texas Sharpshooter Fallacy” is a third way of looking at it which would be more familiar to statisticians.

Steve Horn BSc (Electronic Engineering)
Computer Programmer working in the field of statistics for industry
West Lothian
31 July 2006

I am self-taught in statistics so my observations should be checked by an expert in logic and/or statistics.

I first noticed that the Shirley McKie case could be an example of the “Texas Sharpshooter Fallacy”. An explanation follows:

The “Texas sharpshooter” fallacy

This is a common misuse of statistical information illustrated by a gunslinger taking a quick shot at the side of a barn. He then draws a bullseye target round the bullet hole and claims to be a sharpshooter. If the target had been drawn before he fired the shot then random factors would be extremely unlikely to result in a bullseye hit. However by drawing the target afterwards, he has turned a highly improbable event into a near certainty. All he has to do is hit the barn.

Comparison, Texas sharpshooter and the Shirley McKie fingerprint match.

The Texas sharpshooter paints the bullseye around the bullet hole AFTER firing the shot.
The prosecuting authorities develop a theory (that Shirley McKie was in the murder house) AFTER a fingerprint match is found.

The Texas sharpshooter offers only two explanations for the target hit. Either he is indeed a sharpshooter (he hit the target due to his skill) or a near impossible coincidence has happened (he is not skilful and the bullet hit the target by chance).

Fingerprint experts offer only two explanations for print Y7. Either the print is McKie’s or a near impossible coincidence has happened (despite many points of similarity the apparent fingerprint match is by chance).

The Texas sharpshooter thinks that a significant non-random event has occurred (he hit the bullseye using skill). In fact a non-significant random event happened (he hit the barn somewhere).
The jury in the perjury trial is led to believe that a significant non-random event has occurred (Shirley McKie made fingerprint Y7). But perhaps a non-significant random event happened (somewhere in the world at some point in time there was a large number of similarities between two fingerprints).

If only one suspect was in the area of a murder at the time, and there is one fingerprint on the murder weapon then the target is painted before firing the shot. “If the fingerprints match then we will have found the culprit”

In a traditional criminal investigation there will be a number of suspects and a number of fingerprints. The target is still painted before firing the shot. “If any of the crime scene fingerprints match any of the suspects then we will have found the culprit”. This is a wider target than the first case so it will be a little easier to hit it by chance, rather than skill. Hitting by chance represents an erroneous match due to random factors (bad luck) rather than a match with the real culprit.

Almost any fingerprint test carried out in Scotland could have led to a McKie type accusation. But why limit ourselves to Scotland? A McKie type inquiry could have happened in Amsterdam in 1990 or could happen in Los Angeles in 2020 - as long as the authorities are prepared to pursue a disputed fingerprint all the way to a perjury trial, and the accused maintains innocence. We have not painted any target before we shoot so we cannot use the normal low probability of error as the proof of guilt.

Stephen Horn
26 June 2006

Link to the section on DNA and "The Prosecutor's Fallacy” in the CPS guidelines on the use of Scientific Evidence:

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Have a GREAT week!