Convictions on the DNA database?

3 March 2010

DNA by Mark Cummins.In a major speech on crime Gordon Brown again made the claim that the DNA database plays a significant role in solving crimes.

It is a contentious political issue, with both Conservatives and Liberal Democrats concerned about its fairness and effectiveness.

The European Court of Human Rights last year labelled the DNA database in England and Wales as "blanket and indiscriminate" and the Government must now change the system to comply with privacy laws.

In his speech the Prime Minister said: "We all know how crucial DNA technology is in bringing criminals to justice. Last year alone there were 832 positive matches to the DNA database in cases of rape, murder and manslaughter."

The statistic was one of several that the Prime Minister used to support the Government's record on crime. In a sideways dig at the opposition, Mr Brown commented: "Statistics are important and we need to get them right."

Is the Prime Minister's statistic right and does it support the claim he made?

The source

The National Policing Improvement Agency (NPIA) figures show that in 2008-09, there were DNA matches in 252 homicides and 580 rape cases.

Gordon Brown's statistic is therefore correct. It is the same figure the Home Secretary Alan Johnson used to announce new proposals on DNA retention in November to support the claim that DNA had: "played a key role in the conviction of numerous individuals for the most serious of crimes over the years".

The context

But, what is the significance of a DNA match and what relationship does it have to solving crime?

The NPIA website itself calls into question the meaningfulness of the matches figure. The site explains: "A 'match' means that DNA found at a crime scene matches DNA from a person when loaded to the NDNAD. This includes cases where someone has a legitimate reason for being at a crime scene."

The 832 matches figure used by Mr Brown and earlier by Mr Johnson to support their argument does not relate to the number of crimes solved or to the number of criminals that are brought to justice.

Matches could refer to any number of outcomes including identifying suspects, helping police eliminate someone from their enquiries or being entirely coincidental.

To illustrate the point, if the entire population was on the DNA database then there would be a DNA match for every crime scene. It does not follow that all crimes would be solved.

Detections using DNA

Citing the number of crimes that have been detected involving DNA matches is a more useful analysis than simply referring to the number of cases in which there were DNA matches.

Government figures on detections show that 74 homicide and 169 rape crimes last year involved DNA matches. This figure of 243 serious crimes detected is significantly lower than the 832 matches Mr Brown used to support his claim about DNA bringing people to justice.

But, even this lower figure does not definitely prove the Government's point. The crimes detected statistic does not establish a causal link between the DNA match and the detection of crime. So, even though in these 243 cases the crime was resolved, we do not know what role the DNA matches played in that process.

As the NPIA confirm:  "It is hard to say how many detections have resulted from the use of DNA as every case is different and other forms of evidence will also contribute to detections."

Any convictions?

When pressed in parliament to provide statistical evidence on the number of convictions, Home Office Minister Alan Campbell said: "Convictions are achieved through integrated criminal investigation so it is not possible to say whether the DNA match was the key factor in solving the crime."

Professor Ross Anderson, a database specialist at Cambridge University, explained to Full Fact that matches, detections and convictions are different measures and should not be confused.

"It's remarkable how reluctant the Government has been to produce joined-up statistics linking matches to detections to convictions", he said.

Referring to the detection figures from the year before, Dr Anderson added: "It's ridiculous for the Prime Minister to produce an isolated figure of a number of matches, when his ministers have been unable to say in which if any of the 83 homicides and 184 rapes detected in 2007-8, where a DNA match was detected, the DNA match was the key factor in solving the crime."


There are individual and high-profile cases, including the five murders of prostitutes in Ipswich in 2006, where the DNA database played a key part in solving the crime. But, statistically, the Government has not been able to show the significance of the DNA databases' impact on solving crime.

The Government's own figures show that DNA matches were only involved in 243 serious crimes detected last year, far lower than the 832 figure used.

Both Prime Minister and Home Secretary have used a figure for DNA matches to support their argument that the DNA database has a significant role to play in solving crime. The statistic is correct but it does not support this argument. Without caveat or explanation it has the potential to mislead.

The DNA database now stores over five million DNA profiles from both convicted criminals and innocent people.

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