My decimal dysfunction hell

The Tories are taking a pasting in the press for getting their decimal points in a twist. They erroneously reported that more than half the girls in certain deprived areas were pregnant before the age of  18.

Stop and think about that for a minute. Does it sound right? In your average class of 16-year-olds, are half wearing maternity gymslips? No, because it isn’t right. But I’m all too familiar with the reason for the mistake, and I’ve made it myself.

Ordinary mortals, in which group I include journalists, think in percentages, or in fractions like half or quarter. Epidemiologists and those responsible for collecting large scale population data tend to think in much bigger scales – cases per 10,000, or more. Even more confusingly, researchers tend to report findings from big, long-term studies in ‘life years’ or ‘exposure years’ – so, for example, you might learn hormone replacement therapy carries a risk of one additional breast cancer for every 1,000 woman years of exposure to HRT (I made that stat up, please don’t quote me).

All this takes some disentangling to put in a format that other ordinary mortals, like newspaper readers, can readily grasp. And you know how it is with those slippery decimal points… Which is my excuse for only noticing at the last minute that an article I’d worked on claimed that more than 50 percent of children are now diagnosed with asthma.

Like the Tories’ schoolgirl mums, my asthmatic kiddies were out by a factor of 10. Approximately 50 in 1000 children are diagnosed with asthma, and approximately 50 in 1000 under-18s get pregnant. Five percent, not fifty percent. Oops. This sort of error is incredibly hard to spot if you’re working on auto-pilot. That’s why newspapers have sub-editors, not just to spot typos and write outrageous headlines, but to run a fresh pair of eyes over an article and say: ‘Hang on a minute. That doesn’t sound right.’

Of course, it also depends on your mindset and assumptions. I spotted my error because I don’t see half of the children I encounter puffing on inhalers. Perhaps no-one at Conservative Central Office knows enough teenagers from deprived areas to notice out that half of them aren’t pregnant.

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