UK DNA policy – four uneasy pieces

Some thoughtful challenges to the government’s policy plans on DNA retention have appeared recently. The current policy is under review because of a European Court of Human Rights ruling that the retention of DNA from those who are arrested but not subequently charged breaches EU law.

Article in the Guardian, arguing that the current policy proposals are based on flawed evidence and interpretation;

Paper by two professors from Lancaster University, cited in the Guardian article;

– Blog post on Dr Ben Goldacre’s “Bad Science” blog with some trenchant criticisms of the Home Office research into the statistics of criminal activity;

And here’s the Home Office consultation paper referred to by Dr Goldacre.

Here are a couple of statements I found in these sources, which indicate some of the difficulties of formulating policy statements on the basis of statistical investigation:

“innocent people who have been arrested are as likely to commit crimes in the future as guilty people” – Assertion from the Home Office paper

“half of all crimes are committed by something like 6% of persistent offenders” – comment by Prof. Keith Soothill (University of Lancaster)

I find it hard to see how both of those statements can be true… but then, that’s probably why statistics and I have never really got on.

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4 thoughts on “UK DNA policy – four uneasy pieces

  1. Presumably not all offenders are persistent offenders. So 6% of persistent offenders equates to x% of all offenders, where x% is significantly smaller than 6%. And they are the ones who commit half of all crimes, according to Professor Soothill. (I am sure a respected academic at a respectable university would not be so careless as to say "6% of persistent offenders" when he really meant "the 6% of offenders who are persistent".)In which case, the other half of all crimes are committed by the members of a set comprising:* More than 94% of guilty people* Innocent people who have been arrested* Innocent people who have never been arrested.Furthermore, unless the police are completely incompetent, the set of "innocent people who have been arrested" is not a random sample of the innocent population.So it is perfectly possible to imagine a set of numbers compatible with both of these policy statements.We might also note that the blanket concept of "crime" covers everything from murder to parking tickets. Of course, the trouble with such blanket statistics is that it may encourage our political masters to adopt a single policy for all classes of crime. Let's hope they are wiser than that.No sniggering at the back please.

  2. Robin Wilton says:

    Very charitable of you, Richard… ;^) Both in terms of your kind explanation of the "Venn diagram" and in terms of your interpretation of Prof Soothill's phrasing. I am nasty and sceptical, and wonder if what he meant was , in fact, that 50% of crimes are committed by 6% of people, and that those people are, like Norman Stanley Fletcher, "habitual criminals"…

  3. I have now read Professor Soothill's paper as well as the statement attributed to him in the Guardian (quoted above). The paper is a lot more precise, and a lot more critical of government policy.I think the key point here is not the quality of the statistics, but how any such statistics are deployed in the formulation of policy. Statistics cannot tell us what to do in the future, it can merely help us analyse the effects of our past actions.And proper statistical analysis can help expose flaws in an evidence-based argument for a given policy. When policy-makers try to cherry-pick the statistics to prove a point, it is right and proper for knowledgeable souls (such as Professor Soothill) to indicate that the statistical evidence doesn't all point to the same policy conclusion.

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