Why do single event probabilities confuse patients?BMJ 2012; 344 doi: https://doi.org/10.1136/bmj.e245 (Published 11 January 2012) Cite this as: BMJ 2012;344:e245
- Gerd Gigerenzer, director1,
- Mirta Galesic, researcher1
- 1Centre for Adaptive Behaviour and Cognition, Max Planck Institute for Human Development, Berlin
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: for seven or eight hours.1 Others believe that it will rain tomorrow in 30% of the region, so probably not where they live. In New York the majority believe that it will rain on 30% of the days for which the prediction was made. That is, most likely it won’t rain tomorrow.
A chance of rain tomorrow is a single event probability. It refers to a unique event, such as rain tomorrow, and by definition does not specify a reference class. But people think in terms of classes: time, region, or days. These are not the only ones. As a woman in New York explained, “I know what 30% means: three meteorologists think it will rain, and seven not.”
It is often said that people cannot think in terms of probabilities. But the real problem here is professionals’ risk communication. Using a frequency statement instead of a single event probability, meteorologists could state clearly that “it will rain on …