Dot writes: I’ve just bought and started a book by Jeffrey S. Rosenthal, Struck by Lightning: The Curious World of Probabilities. I was drawn to the topic for a general reason and a specific reason. The general reason is that I think probabilities are bandied about recklessly in the press by people who don’t understand statistics trying to persuade or terrify other people who don’t understand statistics. A made-up but verisimilar example: ‘if you send your child to nursery school s/he has a 20% increased risk of going to Superquinn and shooting everyone’. You recognise the kind of guff. The specific reason is to do with contraception. I was told that the diaphragm (a rather rare method these days) has ‘a pretty decent failure rate’, which my doctor quoted as ‘about 20%’. What on earth does that mean? A normally fertile couple having regular sex only has about a 20% chance of conception in each cycle when not using any contraception at all. (There I go with another statement of probability – but at least that one is a little easier to understand!) If the 20% figure means 20% of women using the method will get pregnant, does that include women using it incorrectly? Over how long a period of time? Or does failure mean sperm getting through, even if they don’t then strike the jackpot? I don’t expect Rosenthal to clear up that particular puzzle, but it was my rather unhelpful consultation on this topic that particularly focused my mind on how such figures are summoned up to influence one’s decision-making, and how little one really learns from them.