This is the sort of thing that Ben Goldacre and Alex Harrowell continually go on about. It\’s all very well having a medical or security check to try and find something, in this case the eeeevil pervs who would monster the kiddies, and there can indeed be some success at finding them. However:
Up to 13,000 people have been wrongly labelled as criminals or accused of more serious offences because of blunders by the police and the Criminal Records Bureau.
There will also always be false positives. Always.
Which leads to something of a problem, for this existence of false positives is something that those designing and proposing such systems are extraordinarily shy of talking about or acknowledging. And the problem is that without revealing, accurately, what this rate is, we can\’t work out whether the system itself is worth having.
Now I know nothing at all about (please do make the "no surprise there" jokes) how many people actually have CRB checks nor how many who do have them are successfully found out and prevented from being eeeevil pervs who would monster our children.
But imagine some numbers (and they are very imaginary). Say there are 100 paedophiles prevented from working with children by this system. Yes, we do think that 1 prevented molestation is more important than 1 person who wouldn\’t do so being prevented from working with children. But more important than 10? 50? 130?
At some point we do have to say that a high rate of false positives is causing more damage than the system is preventing. In logic, at the limit, we might have the entire population falsely identified and thus there is no one to care for the kiddies at all, clearly a worse outcome than whatever small number would be molested in the absence of the system (because, of course, not being cared for by anyone the kiddies would all be dead).
As I say, I don\’t know the real numbers here but the logic still stands. Unless we know the true rate of false positives and unless we can clarify the value we place upon what is being prevented against the costs of those false positives, we cannot know whether the system itself is value adding or value destroying.
I suspect the latter but it is that, a suspicion (and that\’s without taking into account the cost of the system itself. Are we sure that we\’re preventing crimes to hte value of the squiddely millions that people have to pay to get checked?).