The DNA database: innocent or guilty, what's the difference?

 The Home Office has published a consultation paper on its plans for retaining DNA profiles of individuals arrested for crimes but not convicted.

 It argues that, for adults,  it should be allowed to retain the profiles for at least six years – on the implausible basis that innocent people are just as likely as guilty ones to go on to commit crimes. “This is obviously a controversial assertion” the paper admits rather shamefacedly.
 It is much worse than that, says Professor Sheila Bird of the MRC Biostatistics Unit in Cambridge. She says in an unpublished analysis that the arguments used by the Home Office are “a travesty of both statistical science and logical thinking”.
A decision on what to do about the DNA profiles of innocent people was forced on the Home Office by a judgement in the European Court of Human Rights last December. It ruled that the blanket and indiscriminate retention of DNA samples and profiles of people not found guilty of crimes “fails to strike a fair balance between competing public and private interests” and that the present arrangements in England and Wales had “overstepped any acceptable margin of appreciation in this regard.”
The Home Office’s response, justifying retention of profiles for a minimum of six years, amounts to arguing that being arrested is an accurate predictor of the propensity to commit crime. This may be true: but proving it is a different matter – particularly in the light of evidence published in the Daily Mail (4 June 2009) that youths with no criminal records are being deliberately targeted for arrest by the Metropolitan Police so that their DNA profiles can be taken.
The obvious way to test the Home Office’s claim would be to take a large number of DNA profiles from individuals arrested but not charged or convicted, and then to follow up those individuals for several years to see what crimes, if any, they committed. But, in spite of the UK having pioneered DNA fingerprinting, the existing databases do not provide the information needed for such an analysis. There are 500,000 DNA profiles that do not even have a link to a Police National Computer crime number – so the crime for which they were taken has not been registered.
Instead, the consultation paper includes a section written by Ken Pease of the Jill Dando Institute attempting to justify the Home Office claims by a less direct route. He examines 532 DNA profiles obtained by the Metropolitan Police on a single weekday (June 1) in three successive years, 2004, 2005 and 2006. These profiles came from individuals with no previous convictions.
The chances of any of these people being re-arrested for other offences over variable periods of time between 30 and 54 months were then recorded. Oddly, the probability of re-arrest within 42 months (the 2005 cohort) is higher than that of re-arrest within 54 months (the 2004 cohort) – which is the opposite of what common sense suggests.
The table contains no standard errors, which casts doubt on the conclusion it draws that re-arrest rates are identical between those who were subsequently convicted and those who were not even charged.
Even worse, the public is not even allowed to know how many of the original 532 were innocent. The answer is clearly substantially fewer then 532, unless the Metropolitan Police spend most of its time on futile arrests. Hence, the entire consultation probably rests on fewer than 200 innocent profiles. We should be told...
But the analysis then takes an even greater leap into the dark. It uses data on people convicted of a crime in 2001, and followed up for six years, to estimate the reconviction rate of the people who provided the DNA samples. So reconviction rates for guilty people were taken as a likely pattern for conviction rates for a group of individuals with no previous convictions.  
The reconviction rate is known for only six years, but the paper confidently extrapolates that to 25 years, using a power function. From this it concludes that only by retaining DNA samples for 24 years could one be certain not to miss any possible convictions of the original group. By keeping them for at least six years, half the potential crimes will be missed, the analysis concludes.
Whatever became of the idea that you are innocent until proved guilty? Under the rules operated by the Metropolitan Police, everyone is a potential offender whose DNA must be stored in case they commit a crime. In Camden, a total of 386 under-18s had their DNA taken and stored by police last year, according to figures obtained by Jo Shaw, the LibDem parliamentary candidate for Holborn and St Pancras, under the Freedom of Information Act.
An (unnamed) officer told the Daily Mail: “We are often told that we have just one chance to get that DNA sample and if we miss it that might mean a rape or murder goes unsolved in the future.”
The consultation closes on August 7.