On today's Thought for the Day, John Bell railed against the evils of drink:
What do you think would happen if it were discovered that 50% of the prison population in Britain had been helped on the road to crime because of one identifiable food additive which had an adverse effect on behaviour?...So why do beer glasses not have a sign which states: 'Alcohol may lead to criminal activity of a violent nature, dangerous driving and lasting damage to the liver.'?
Mr Bell might not be abusing drink, but he's abusing statistics. Let's rejig the numbers. On one estimate, there are 9 million problem drinkers in the UK, out of an adult population of 48 million. Let's assume that all the prisoners Mr Bell describes are drawn from this 9m. This is reasonable. After all, if drink lands you in chokey, you've a drink problem.
Now, last week there were 77,774 people in prison. On Mr Bell's figures, 38,887 of these have a drink problem and 38,887 do not.
So, we can say that the proportion of problem drinkers in prison is 38,887 in 9 million. That's 4.3 per thousand. The proportion of non-problem drinkers is 38,887 in 39m, or 1 in a 1000.
Now, you can argue around these figures, but I reckon they're ball-park OK.
So, you could say problem drinkers are more four times more likely to end up in prison than non-problem drinkers. Or you could say that 996 in 1000 stay out of prison.
I don't think either figure justifies Mr Bell's complaints.
What's more, I suspect there are many behaviours more correlated with crime than heavy drinking; Theodore Dalrymple once claimed that all criminals were smokers. And what about tattoos?
I mention this not merely to be an anorak. We are approaching the 242nd birthday (on November 10) of the original statement of Bayes theorem; it's this pdf. It seems the statistical prowess of our clergymen has not improved in this time.
ad hic argument?
Posted by: dearieme | October 27, 2005 at 03:37 AM