when I read this it struck me that by that arithmetic the chances of any 2 people having the same birthday in a class of 20 would be 20/365 rather than about 50/50 as it really is…

“They will be monitored to ensure that children attend school, go to bed on time and eat proper meals. Private security guards will also be sent round to carry out home checks, while parents will be given help to combat drug and alcohol addiction.”

www.express.co.uk/posts/view/115736/Sin-bins-for-worst-families

It does make the point though that matches made on less than the normal ten loci have a higher risk of false positives.

Does anyone know how big these loci are? They must be more than one base pair long (4^10 is just over a million).

It also makes the point that a new technique, LtDNA, can use up the sample in getting its match, meaning that if the only match is a false negative further testing on the sample to exonerate the innocent suspect is not possible. I have to admit this is pretty scary.

Having a universal database would still dilute the effect of this on the jury – the question would be; would you get more innocent people convicted as a result of this (due to crimes committed by people who were not on the database) than you would acquit from the realisation that the DNA evidence corresponded to multiple potential suspects and as such could not be relied upon?

This sort of risk analysis does come into play at the level of the jury, rather than at the investigation level, where the data will show police which suspects to concentrate on and, potentially, obtain confessions from criminals who like the jury believe that DNA is incontrovertible – but not from the innocent who know it can’t be because they didn’t do it. Yes, this means that the innocent get questioned, but that’s pretty much the accepted situation with regard to police investigations.

Suw – you were concerned about the alternative uses of the DNA data. I agree wholeheartedly with you, and am most certainly NOT an advocate of the current system where the entire DNA sample gets kept. Another poster made the point that the more DNA you have the more you reduce the risk of a false positive, but there is nothing to stop you checking as many base locations as necessary on your test sample and your suspect once you have identified him – you don’t need to keep the sample for this.

If only the very small amount of DNA needed for the initial marking technique is kept then data misuse is not a major issue – it is currently useless to insurance companies. You can make a good guess at ethnicity from it but I’m not sure how useful this will be to anyone. The only “naughty” use I can think of is proving someone was somewhere legitimately, but privately – for use in blackmail. It wouldn’t help the tabloids, even if it did get out, as it would be illegal for non-law enforcement agencies to use this data.

David Mery – you mention a situation where someone got arrested and had his home searched despite the presence of previous DNA evidence. I agree this is pretty stupid, but it is n=1; we’re presumably agreed on the dangers of drawing conclusions from such evidence.

SteveGJ – you’re absolutely right; my logic was flawed (or at least it didn’t convert very well into prose; it was so long ago I can’t remember now )

The fact remains that if there are multiple matches it would be better for the jury to know about them, which may or may not be the case with the current system, but should (in the majority of cases) be the case with a universal database.

SteveGJ also suggests that forcing citizens to do something against their will is a very dangerous line to take. I’d argue that, rather than being forced to do something against our will, agreeing to do things that we would rather not do is a price we pay for living in society, with its privileges that we would rather not do without. A database of DNA markers only is minimally dangerous and can provide large benefits.

A carte blanche to investigate email is considerably more dangerous (and is, I believe, pretty much done anyway by the USA via Echelon; its not illegal for THEM to spy on UK citizens, and not illegal for them to share the information they get with the UK authorities).

Bish – yes, a DNA database could cause some convictions to be quashed, but only the unsafe ones which relied on DNA matches alone in cases where there was more than one match. One would also expect it to bring many more criminals to justice.

FergusKane is concerned that one might get the right match on the wrong sample. However I’d argue that the average jury should be able to understand the logic that a skin sample, for instance, ties a person only to the site and not to the crime. Certain samples are harder to explain away than others – skin samples under the nails of the deceased, for instance.

Bagpuss wonders how we will get a sample from “everyone”. I agree that a truly universal DNA database is not feasible – foreign visitors would as (s)he says likely remain exempt in the current climate. Posing for another could be made tricky by following up apparent duplicates, although this would admittedly increase the expense of the project. Taking a photograph of people at the time of sampling would reduce the cost (on the basis that if you have a match between a 5′ model and a 7′ wrestler you probably don’t need to follow it up) but would probably double the outcry. Getting the DNA sample at birth or entry into the UK would reduce the risk of people slipping through the net.

Miscarriages of justice such as the one you describe can still occur, but at a rate of less than one in a billion. Assuming a hundred thousand cases in the UK per year which use DNA evidence this would be one case per ten thousand years (I’m sure SteveGJ will check my math )

Compare this to the potential 1 in 2 miscarriages of justice currently taking place when one relies on a jury.

But more importantly, you are making the assumption that the person paid will also be on the database under their own right. If they’re currently living in the country without the authorities knowing, or are a temporary visitor, then they won’t be.

My post wasn’t intended to be an accurate description of what would happen if a universal database were implemented. It was intended to point out just a few of the more obvious possible and probable flaws that will make it impossible for a database of “everyone” ever to exist. Even if that particular one is impossible in practice (and I doubt that) it doesn’t detract from my fundamental point.

CSI Myths: The Shaky Science Behind Forensics

The truth about 4 common forensic methods: Problems with fingerprinting, ballistics & fiber analysis forensics

So how does this guy get away with not giving a sample when it’s really his turn – presumably a match for someone already on the system would be flagged ?

Your entire argument is based on the assumption of holding the DNA of everyone, but who is this everyone and how do we ensure that we obtain a DNA sample from them?

I think it’s generally agreed that we do not have a comprehensive list of everyone living in this country. So how do we identify everyone, including those who are likely to be reluctant to make themselves known since they’re here illegally, and persuade them all to come forward to donate a DNA sample? And even if we could somehow achieve this miracle, there will undoubtedly be a way for people to appear to be giving a sample, but to actually give a fake one – say by paying someone else to pose as them.

And that’s before we even consider people coming to the country on a temporary basis – tourists, business travellers, etc. Are we to stop everyone at the border and demand a DNA sample for the database? That might have a rather serious impact on the tourist industry and on business in general.

So, at the “best”, we’ll only ever have an almost-complete database. Criminals will, of course, have the greatest incentive to find some way of avoiding their DNA being on the database, so the holes in the database are likely to be significant, even if they’re only small.

So now, a DNA sample is taken from a crime scene, but it only matches to one person on the database. This person is, in fact, innocent but we know that they must be guilty, because they’re the only match on this “complete” database. Of course, the real culprit was someone who is already back home in some other country, or is the bloke who paid someone a few quid to impersonate him when his sample was taken, or is someone we have no idea has been living here for the last 5 years. Unfortunately, the innocent match was at home asleep, alone, at the time the crime was committed. They happen to live reasonably close to the crime (hardly unlikely – pretty much everyone in London, and plenty in the surrounding counties would live sufficiently close to a crime in London) and has absolutely no way of proving that they weren’t actually raping someone on the other side of the city.

So we lock them up and throw away the key, while the real culprit goes on happily committing more crimes. And the innocent man gets to enjoy Her Majesty’s hospitality, with no-one even questioning his guilt unless the real culprit is careless enough to leave his DNA lying around at a crime scene again, while our innocent guy is inside.

The essential problem with a national DNA database, is not that of a false positive match. I believe the real danger is of finding a true match, but with the wrong samples and leading to the wrong conclusions. If every UK citizen is in a DNA database, then one may assume that any hair fibre, skin sample or blood spot in a crime scene will match to someone. The problem is, did that person commit the crime?

A national DNA database, would reduce the emphasis on real policework. A DNA match is a MUCH more powerful finding if the suspect is identified BEFORE the match. If the suspect is identified based on a match, then the rest of the evidence can be fitted around the DNA evidence. The onus then falls upon the suspect to provide an alibi. I’m not suggesting that fitting of the evidence would be done deliberately, but the DNA evidence would inevitably result in a bias in the investigation.

In the end, a jury may be asked to make a decision largely based on DNA evidence from a search of a national database. Given the state of understanding of science and statistics, not only in the general public, but also within the legal profession, this is not a pleasing prospect.

On a slightly different tack, it has been said that private citizens should always work to minimise the power of the state. I think this sentiment is generally right, and should be applied here.

Mike, you are dead right. The other giveaway, of course, is that the post has cock-all to do with the topic under discussion.

I guess that was her “Keeping the Right (Wing) People Out” report.

Government departments have a reputation for losing large chunks of data…the lost laptop effect….so how long before the partial data ended up in somewhere else. Also what assurance would we have that only the partial sequence would be stored?

If the full sequence is stored then i’m sure that health insurance companies would have a keen interest in the data. With the researchers of various countries finding markers for various ailments the human body can fall victim too this data could become very valuable…so how much am i bid for this cough cough *lost* cough laptop?

In case anybody ask’s my DNA has been at least partially sequenced in a medical study organised through the british blood service and i have no idea where the data ended up.

Does anyone remember Jacqui Smith criticising Professor Nutt for having the bare-faced cheek to state the plain fact that horse-riding is more dangerous than cannabis? They are simply not interested in having that ‘open and honest debate’ they are always going on about.

Ignoring for the moment the other retorts posted above, and simply taking your reasoning and maths at face value, another problem emerges – if assembling a comprehensive DNA database of the entire country has the effect of dramatically reducing the number of convictions attained solely or largely on the back of DNA evidence, this begs the question – then why spend so much public money on doing it in the first place? If a comprehensive database would make crimes even harder to prove, what would be the use?

Back on BG’s original column; it’s alarming – but not exactly surprising – to see the Civil Service chucking together reports that appear to support the government’s position. It often seems the question isn’t whether or not we should have DNA databases, ID Cards and Telescreens, but rather when they can squeeze them into existence without too much fuss; using Iraq as a template, the primary tactic is to assemble as much incomprehensible and/or misleading ‘evidence’ as possible, and wait until the public acquiesce from exhaustion and a sense of hopelessness. When you have government ministers asserting (and perhaps believing) that “the first duty of the state is to protect its public”, it’s not at all surprising that they’ll bend the rules of good practice to ensure that their best intentions are realised…

All the same it’s nice to see someone flagging up the gibberish, for what good it might do.

At arrest, two samples are taken (usually buccal scrapes). Those are sent to one of three private labs under contract with the National Police Improvement Agency, which is the custodian of the National DNA Database (NDNAD). The labs analyses one sample and derive a DNA profile, the other sample is put in a freezer. The DNA profile, a series of 20 numbers plus sex information is uploaded to the NDNAD.

Samples are kept in the labs freezer and profiles on the NDNAD. (There are also mentions of it on the Police National Record.) These are kept for either forever or until the subject reaches 100-year old (my reading of the step-down model is forever, but some contracts may state a 100-year retention period), anyway the DNA profiling technique is too young to figure that one out yet.

To find out all the info kept in the NDNAD, check out Information retained in the National DNA Database profile records.

There are many differences between retention of DNA profiles and fingerprint. Here’s an example. Some research (without consent) was done on both DNA samples and DNA profiles. One such research that was exposed via FoI requests a year or two ago by GeneWatch UK concerned attempts to predict visible ethnicity from DNA profiles. The DNA profiles supposedly contain only what is called non-coding information, however allegedly the research did find some correlation (though I don’t think it was ever peer-reviewed). Other reasons include that the database has a racial bias. All this is probably trumped by the fact that there’s no evidence it is helpful to retain profiles on such a scale. When the database doubled in size, the number of detections didn’t increase. Ben wrote about the needle in haystack problem, here’s a post about how this is an issue in the NDNAD context: Growth of the National DNA Database increases the risk of miscarriages.

For a rapid overview of the NDNAD, check out the Reclaim your DNA website.