DNA profiles: another police scare story

On Monday, senior policemen giving evidence to the House of Commons Committee examining the Protection of Freedoms Bill said that 1,000 crimes a year would go undetected if its provisions for retention of the DNA of innocent people became law.
The evidence was reported in The Times under the headline “Rapists ‘will go free’ under DNA reform”. The use of inverted commas in a headline usually means it is quoting a claim rather than a fact, but in this case the claim had not even been made.
Chris Sims, Chief Constable of the West Midlands, who leads for the Association of Chief Police Officers on forensic science, had indeed said that the plan to remove innocent profiles would cost 1,000 convictions a year, but had been unable to say how many of these would be serious crimes such as murder or rape.
The evidence given by Mr Sims and others disclosed that 20 per cent of the DNA profiles on the database come from people who have been arrested but not subsequently found guilty of any crime. Since the database contains 6 million profiles, that is 1.2 million. These 1.2 million profiles achieve 1,000 matches a year with crime-scene profiles, Mr Sims explained. (This is not the same as convictions, as only a minority of matches lead to convictions.)
He did not explore, nor did the MPs ask him, how many matches would be expected from 1.2 million profiles taken from a random sample of the public matched for age and sex with the 1.2 “innocent” profiles. What his evidence amounted to was the self-evident claim that the more profiles that are on the database, the more matches will be achieved.
The legislation, which comes in the wake of the S and Marper case in the European Court which declared DNA retention rules in England illegal, would remove profiles from those arrested for minor offences but retain for up to five years the profiles of those arrested for serious offences.
In May 2009, as the then-chair of Home Office’s Surveys, Design and Statistics Subcommittee, one of us (SMB) wrote an advisory letter to the then-permanent secretary on the types of statistical analysis that could, and should, be brought to bear on the National DNA Database if empirical evidence were to be adduced in defending any new rules on retention of “innocent” profiles.
This week’s evidence session suggests some attention has been paid to the issues raised but perhaps not enough – even though more than two years have elapsed since the S and Marper ruling.
In their evidence, senior police officers echoed still the illogical party line that Scotland’s rules against retention of “innocent” DNA profiles were not evidence-based before advancing evidence for England on the likely impact of Scotland-like rules. But they did come up with two figures: the 20 per cent of England’s DNA profiles that pertain to those who have never been convicted and the 1,000 matches that would be forfeited under the proposed rules.
Notice, first, that both of these were quoted in ‘round’ figures which is generally a clue that some form of estimation, rather than actual counting, underlies them. Secondly, officers were surprisingly unable to state how many of the forfeited matches pertained to serious violent offences, which is a second clue to estimation rather than actual counting. Clearly, had the 1,000 new crime-scenes actually been identified, for which “innocent” matches would have been foregone, officers could have answered MPs’ questions on how many related to serious violent offences.
The evidence session was fascinating for another stone unturned – how DNA samples are identified when forwarded to those (soon to be privateers) who provide forensic services. Alarmingly, the evidence session read as though personal identifying information such as name and date of birth are provided, rather than a barcode label which only the issuing police force could decode. Little wonder that the Information Commissioner is taking a close interest in data protection matters related to National DNA Database – good practice should apply to the labeling of all DNA samples irrespective of whether or not the source individual is eventually convicted, is “innocent” or is a victim.
Two issues pertaining to the National DNA Database that this week’s evidence session did not cover were:

  1. How many more crime-scenes would scene-of-crime officers (SOCOs) need to attend, or be permitted to spend more time at, to achieve an improved level of  additional matches – because you can’t match DNA samples if SOCOs were either not sent to the scene or tight budgeting limited how long they could spend there.
  2. How many highly prolific offenders (those whose DNA sample has been located at multiple unsolved crime-scenes) are there and how many, per annum, have their set of crimes reviewed by a super-sleuth and are thereby ‘detected’.


Declaration of interest: SMB is a member of Home Office’s Surveys, Design and Statistics Subcommittee but writes in a personal, professional capacity