The news reader announces a 30% chance of rain tomorrow. Thirty per cent of what? Most people in Berlin think that it will rain tomorrow 30% of the time. Others believe that it will rain tomorrow in 30% of the region. In New York the majority believe that it will rain on 30% of the days for which the prediction was made.

This is the (edited) opening of BMJ 2012;344:e245, a fascinating paper about single event probabilities by Gigerenza and Galesic. It’s only short, quite fun, and may convince you to change your practice. Well worth a read.