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The DNA database and you

How big is it? How many get off it? Your questions answered...

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Special Report The National DNA Database (NDNAD) keeps growing: it now holds more than five million DNA profiles of individuals. Getting off the database, if you have been sampled by England or Wales forces, remain as unlikely as ever. And it remains difficult to make sense of the stats bandied at us, with the press quoting wildly differing figures. So we decided to investigate.

In August, the Daily Mail reported that "4.5 million genetic profiles [are] on record. Up to 1.5 million - or a third of these - are from innocent people".

In another article on the same day, the Mail reported "[t]he figure of 573,639 people on the database who have not been convicted, cautioned, formally warned or reprimanded has pushed the overall total to 4.2 million."

This is an extreme example of the difficulty of making sense of statistics concerning the NDNAD. Our first step was to find source data.

In May 2007 the National Policing Improvement Agency (NPIA) started to administer the NDNAD. We'll use data obtained in a recent response to a Freedom of Information request to the NPIA to get some sense out of the data and figure out what are all the implied assumptions.

Data from the NPIA is authoritative, but the organisation's view of what is the NDNAD is a matter of opinion. The NPIA claims that "The NDNAD is not a criminal records database. It holds very little information about a subject's identity - only their name, date of birth, sex and ethnic appearance. Inclusion on the DNA database does not signify a criminal record, and there is no personal cost or disadvantage by being on it".

The National DNA Database Ethics Group and the Human Genetics Commission both consider the NDNAD to be a crime-related intelligence database. Recent findings suggest that composite statistics do not mask identity within genome-wide association studies and that DNA profiles previously considered anonymous and not containing genetic markers may reveal much more than was thought.

Often assumptions are made about the data; and these may not be the same for different sets of data. The NDNAD includes profiles of DNA samples taken by forces from England, Wales, Scotland and Northern Ireland, but often figures given in Parliament or in the press are only for samples taken by the England and Wales forces.

The NDNAD includes DNA profiles of the DNA samples taken from individuals and profiles of the DNA found at crime scenes. Here are figures up-to 2008-09-01.

(At 2008-09-01) England & Wales forces Other forces
Total number of subject profiles 4,969,225 327,088
Estimated total number of individuals 4,319,807 273,358
Total number of crime scene profiles 320,335 13,749

The subject profiles consist of both profiles of DNA samples taken from individuals following arrest for a recordable offence, known as criminal justice samples, and profiles of subjects who volunteered a DNA sample (whether those that do so are sufficiently informed before they give their consent is an issue that was raised during the presentation of the Nuffield Council on Bioethics; the NDNAD Ethics Group has been discussing the volunteer consent form for DNA sampling and accompanying information), for example, for elimination purpose.

Another source of confusion is that the number of subject profiles on the NDNAD is higher than the estimated number of individuals on it. This is often misrepresented. It happens because some of the profiles held are replicates. Multiple samples are taken from the same subject and profiled when on different occasions there's confusion concerning the person's name. Replication also happens when the police decide to resample an individual. The number of replications is estimated at around 13 per cent (it varies over time and between police forces).

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