Life’s lottery? The trouble with statistics

Simple, neat statistics abound in the media. But, from HIV tests to infant death, figures are often more complex than they look, and misinterpretation can have devastating consequences

In  a national lottery, the sequence 1-2-3-4-5-6 is as likely to tumble from the machine as any other combination, yet picking this combination intuitively feels less likely than a wider spread of numbers.  Photograph: Joel Saget/AFP Creative/Getty

In a national lottery, the sequence 1-2-3-4-5-6 is as likely to tumble from the machine as any other combination, yet picking this combination intuitively feels less likely than a wider spread of numbers. Photograph: Joel Saget/AFP Creative/Getty

Statistics are ubiquitous, used to convey information about everything from money markets to medicine. With their clear scales and round numbers, they appeal to our intuition and seem simple to grasp. Yet this veneer of simplicity is often misleading.

Imagine you’re given a HIV test, which you’re told is 99.99 per cent accurate. It comes back positive. So what are the odds you have HIV? Instinct tells most of us that we almost certainly have the disease. Yet the actual answer is closer to 50 per cent for most patients.

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