Suppose that, in a long (conceptually infinite) sequence of weather forecasts, we look at all those days for which the forecast probability of precipitation was, say, close to some given value ω and (assuming these form an infinite sequence) determine the long run proportion p of such days on which the forecast event (rain) in fact occurred. The plot of p against ω is termed the forecaster's empirical calibration curve. If the curve is the diagonal p = ω, the forecaster may be termed (empirically) well calibrated.

Between 1985 and 2005, boomsters made 10-year forecasts that exaggerated the chances of big positive changes in both financial markets (e.g., a Dow Jones Industrial Average of 36,000) and world politics (e.g., tranquility in the Middle East and dynamic growth in sub-Saharan Africa). They assigned probabilities of 65 percent to rosy scenarios that materialized only 15 percent of the time.