Our local lads of a criminal bent can sometimes be a little slow to catch on to the latest developments in policing. Take young (18) Dwayne, who broke into a house in a comfortable suburb on my patch, and stole the keys to the 11-registered quality German sports coupé that was sitting on the drive. Fifteen minutes later the crew of a patrolling police car (yes they do exist, but often at unsocial hours) were suspicious when they saw a young black boy at the wheel of fifty grand's worth of Stuttgart's finest heavy metal, and tried to get him to stop. He managed to get a mile or two before the inevitable crash, but given the disparity in pace between an Astra and the stolen car, he had run off (or decamped as the Old Bill call it) before the officers arrived on scene.
Sadly for Dwayne, the crash had caused the airbag to deploy, and being smacked in the face by one of those is a forensic dream, providing skin flakes, hair, snot, saliva and more.
The culprit's name soon emerged from the computer, and he was arrested at home, protesting his innocence.
The prosecutor said that the DNA captured gave a billion-to-one trace to our man.
Nevertheless, he pleaded not guilty, and applied for bail.
A jury will have to decide on the former, As for the latter - not really.
This may or may not be a "pure" DNA case. But pure DNA cases require extreme caution. The "billion to one" that is quoted is what is known as the "match probability" - what is the probability that the profile taken from the airbag would match the profile of a person picked at random from the general population who had nothing to do with alleged crime. Of course, with 7 billion or so on the planet, that means that the most likely scenario is that there are 8 people alive who match this profile: 7 by chance, and 1 because he or she was the driver. In other words, the DNA profile on its own cannot suffice to show that Dwayne was the driver - taken in isolation, it actually says that it is 7 times more likely that the driver at the time was someone other than Dwayne....
ReplyDeleteOf coure, it only takes a shred of supporting evidence to radically alter that proposition.
Indeed. The error rates of this kind of test are often cited (even in court) with no understanding of what they mean. I would hope magistrates are trained in what they mean, but I doubt many jurors understand.
DeleteIn this case, the fact that he lives nearby is probably enough additional evidence to justify a conviction, though.
You really have no idea about statistics at all, have you.
DeleteThe problem is the simplistic way experts explain science to non scientists. They say a billion to one to try and make it easy for magistrates etc. If you have a DNA match that sample can only have come from them, barring indentical twins not even another family member would give you a false positive never mind 7 random people in the world. A billion to one is quoted because although two people with the same DNA have never been found you cant say it will never happen.
DeleteThe issue with pure DNA cases is how did the DNA get there? Was that cigarette butt walked into the scene on someone's shoe etc...
Firstly, no worries about airbags and the chain of evidence: the things are pretty DNA clean until they smack Dwayne in the face, and they are not usually trodden on or cleaned up after that. They can acquire additional DNA from the manufacturers or rescue workers etc (who should be on file and easily excluded), but Dwayne's DNA can't get there by magic within a few hours of an accident-free car theft.
DeleteIf the technique was a standard 16 alleles, not mitochondrial, and not Y chromosome, then the 1 in a billion (which sounds very approximate) is still beyond a reasonable doubt. The stranger being 7 times more likely than Dwayne is a statistical fallacy because of the sample size. If the entire world were in CODIS then you would have 7 or 8 hits, and those could be then be narrowed down. However, the sample size is smaller and strangely, just by chance, it includes one well-known local oik spotted by some cops in a stolen car. The probability actually goes down because the sample (relatively small compared to the world population) just happened to contain the match.
Second, tell Dwayne that his defence to the DNA is to provide a mitochondrial DNA analysis. If young Dwayne's brother is of the same ilk, then a mitochondrial match (or a match to Dwayne's Mum) off the airbag, or a mismatch, makes it incontrovertible one way or the other. Were identical twinning to be offered as a defence, then tell Dwayne he needs single nucleotide polymorphisms (SNP) to mismatch. Those studies can even identify which of identical twins the index sample came from. These are the only ways to counter something with this sort of absence of reasonable doubt, and would be scientifically reportable in the journals (because it has never happened).
FYI:- Identical twins' fingerprints differ by the same amount as ordinary siblings.
Net, to be innocent, fleet-of-foot Dwayne has to get to the accident site faster than an Astra, and wipe his face all over the already-deployed airbag before the rozzers arrived !
For reference: https://en.wikipedia.org/wiki/Prosecutor%27s_fallacy
DeleteAnon @1 July 2013 10:56
DeleteYou do not understand how DNA analysis/matching works. The test is a sample of a few locations. It does not constitute "a DNA match" because the entire DNA is not compared.
Why do people who know so little speak with such certainty? Wikipedia is right there to look things up.
What was the reason for no bail? If I understand correctly "there isn't a cat in hell's chance he's innocent" doesn't count.
ReplyDeleteOf course we followed the Bail Act, and our decision was informed by his criminal record that was before us. 'Strength of the evidence' is one factor that we are allowed to take into account. In this case there was much more than that.
DeleteJust to expand your net of those "a little slow to catch on to the latest developments", consider all the pols who seem not to realize, often at the most embarrassing moments, that any nearby microphone is likely to be hot (i.e. "on").
ReplyDeleteOoops!
Police being racist innit,
ReplyDeleteJaded
Should he get legal aid?
ReplyDeleteSure.
DeleteMainly as a point of principle. At the moment he's innocent in the eyes of the law and while it's a waste of money in this case, it would set a bad precedent if we could pick and choose who gets justice.
There are two other practical matters - while he committed a pretty serious crime, he should get exactly the punishment it rates and no more.
Also it may well save the public some money. If he has a hopeless case any sensible legal advisor will point out to him that he would be better off pleading guilty saving a lot of time for everyone.
"were suspicious when they saw a young black boy at the wheel"
ReplyDeleteIs colour important?
The graph here answers your question.
Deletehttp://www.bbc.co.uk/news/uk-16552489
I am reminded of the tale from about five years ago of an expensive German sports car parked in a wealthy suburb of Chicago which was targeted by a gang of hoodlums. They reckoned the could break in and start it – but of course alarms would sound. So the plan was to rent – not steal, rent – a furniture truck and have it round the corner, Then they would drive the car up the ramps and drive the truck to somewhere well away where they could deal with the alarms. The idea was to be away before the Windy City’s finest could be on the scene.
ReplyDeleteBut it all went wrong. The neighbours heard the alarms and phoned 911 and when the police got there the car and the hoodlums were still there; the car blairing away and the hoods arguing with each other.
The problem which had not occurred to them was that the car was a manual and none of them had ever driven one in his life . . . spoilt lot, those Americans!
If you're going to talk about 1 in a billion wrt the rest of the world, i.e. 7 potential matches, well that would only apply if all 7 billion had access to the car in question, right? Practically, it makes it much more certain than that. Besides, they could simply do an analysis of his mouth bacteria etc, and THAT would prove conclusively that it was him, because NOBODY has all the same types of bacteria (numbering in the thousands) in their mouth as anyone else; allied to the DNA analysis, that's enough to throw away the key.
ReplyDeleteAs an aside, don't you just love multiculturalism? And the chance for an 'underpriviledged' **** to be able to relish the joys of driving - erm, crashing - a car far beyond his station, eh. Personally, I would much rather he had taken, sorry, 'borrowed' a Roller or some such, so that those, in the upper stratosphere of their ignorance, could appreciate the finer things in life at an almost sub-human level...