Have you ever been taught the point about correlation and causation?
Without blinking at the contradiction, the prime minister could boast that crime is down by a third, violent crime by 40%. And yet he boasted of the steep rise in prison numbers, up from 60,000 in softie Tory times to more than 80,000 now, and 96,000 soon – presumably regardless of future crime rates.
Correlation is when we note that two things are moving in tandem. However, we don\’t know whether in fact they are linked. It could be that they are not (global warming coinciding with there being fewer priates is one example), it could be that they are both the symptom of some other matter taking place or it could be that the movement in one of the things being studied is the cause of the other thing. We might point to the correlation between electricty blackouts and a boom in births 9 months later as an example of cause and effect….our causation if you wish.
Now, I don\’t insist that there is indeed causation in those crime numbers. It could indeed be simple correlation. But it\’s difficult to call it a "contradiction" unless we explore whether there is in fact some causation going on here.
More people are going to jail, crime is falling. You know, there is in fact an argument that could be made there to show some causation.
Of course, no truly wonderful Polly column could be complete without a mangling of statistics to add to a failure of logic:
In London, he says, murder has declined slightly – from 2.6% per 100,000 people in 2000 to 2.2% in 2007.
There\’s a disturbing sense there that Polly doesn\’t in fact understand percentages. 2.6% means 2.6 per hundred…..that\’s what percentage means. Murder statistics (in order to get away from using 0.026 %) are expressed as a number per hundred thousand. The way she\’s presented them there we\’ve actually got a murder rate of 2,600 people per hundred thousand people….something like the level of deaths in frontline troops in a bloody war (erm, roughly 100,000 in the British Army, some 100 deaths so far in Afghanistan and Iraq……yes, a bloody war).
Well done Polly!