From Adam Lowy
Eric Kvaalen might have scared a few more people than he needed to (Letters, 8 November, p 32). Saying that condoms only work “85 per cent of the time” isn’t very informative without saying what is meant by “time”. If they failed to prevent pregnancy on 15 per cent of the occasions on which they were used, they would be pretty well useless as contraceptives. In fact, in typical use they fail in about 15 per cent of woman-years. In other words, a woman runs about a 15 per cent risk of becoming pregnant in a year in which she and her partner use condoms “typically”. If condoms are used correctly (which simply means following the instructions on the packet) that value falls to about 3 per cent.
Berne, Switzerland
