If a meteorologist says there is a 70% chance of rain tomorrow, that forecast cannot be judged, but if she predicts the weather tomorrow, and the day after, and the day after that, for months, her forecasts can be tabulated and her track record determined. If her forecasting is perfect, rain happens 70% of the time when she says there is 70% chance of rain, 30% of the time when she says there is 30% chance of rain, and so on. This is called calibration. It can be plotted on a simple chart. Perfect calibration is captured by the diagonal line on this chart:
