Enlightenment Now: The Case for Reason, Science, Humanism, and Progress

by Steven Pinker

Tetlock to think aloud, reported that he began by estimating the base rate: he went to Wikipedia, looked up the list of Islamist terrorist attacks in Europe for the previous five years, and divided by 5, which predicted 1.2 attacks a year. But, he reasoned, the world had changed since the Arab Spring in 2011, so he lopped off the 2010 data, which brought the base rate up to 1.5. ISIS recruitment had increased since the Charlie Hebdo attacks, a reason to poke the estimate upward, but so had security measures, a reason to tug it downward. Balancing the two factors, an increase by about a fifth seemed reasonable, yielding a prediction of 1.8 attacks a year. There were 69 days left in the forecast period, so he divided 69 by 365 and multiplied the fraction by 1.8. That meant that the chance of an Islamist attack in Western Europe by the end of March was about one in three. A manner of forecasting very different from the way most people think led to a very different forecast.