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Book review

Reckoning with Risk. Gerd Gigerenzer. Penguin Books Ltd, London, 2002. 250pp, plus glossary notes etc, £14.95.

This is a great book, and you should read it. Yes, it does give a bit of a puff to Bandolier , but that's on page 239 out of 250 pages of text, so clearly it grabbed our attention. It is readable, and a bit of fun, and concentrates on making numbers understandable. If you have had trouble understanding the numerical results of trials or especially diagnostic screening tests, this book will demystify it for ever.

Gigerenzer is the Director of the Centre for Adaptive Behaviour and Cognition at the Max Planck Institute in Berlin, and former professor of psychology at Chicago University. He starts with the premise that, whatever we claim we think we know, when you test us, we don't. Throughout he gives results of tests on how well doctors understand information given to them.

Unsurprisingly, most times doctors haven't a clue, and most of them misunderstand results presented in standard forms using probabilities. But give them frequencies and a simple tool to help them think about what's actually going on, and almost all of them get it right. There are references for all this, so we can chase up the papers and read them in detail.

And there are examples, including breast cancer screening, informed (or uninformed) consent, AIDs counselling, wife battering, expert witnesses who aren't expert, DNA fingerprinting, and violence. Some of them might make you sit up and think. Bandolier thought those on breast cancer screening and DNA fingerprinting particularly illuminating.

So what's the secret? Gigerenzer uses "natural frequencies". He proposes a simple diagrammatical representation of what is happening to work out what is going on. For breast cancer screening, for instance, he helps us recognise that t a woman screening positive on mammography has only a 1 in 10 chance of actually having breast cancer. The whole chapter should be taken and reproduced for women considering screening, and the professionals who have to advise them.

Medical writers, from academics through editors to journalists, should read this book and keep it by their side. Bandolier certainly will. It ranks alongside the Economist style guide as an essential tool of the trade.
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