TerrorismThe historical and future probabilities of 9/11-size terrorist events

Published 11 September 2012

On the eleventh anniversary of the 9/11 attacks, two statisticians apply statistical methods to try and accurately estimate the probability of a 9/11-size terrorist attack occurring during the next decade; examining the historical data from 1968 to 2007, they show that the likelihood of a 9/11-size attack occurring within this time frame was between 11 and 35 percent; looking forward, the likelihood increase to between 25 and 50 percent – and, under certain circumstances, to 95 percent

Quantities with right-skewed distributions are ubiquitous in complex social systems, including political conflict, economics, and social networks, and these systems sometimes produce extremely large events.

For instance, the 9/11 terrorist events produced nearly 3,000 fatalities, nearly six times more than the next largest event. Was this enormous loss of life statistically unlikely given modern terrorism’s historical record?

Aaron Clauset of the Santa Fe Institute write that accurately estimating the probability of such an event is complicated by the large fluctuations in the empirical distribution’s upper tail. They present a generic statistical algorithm for making such estimates, which combines semi-parametric models of tail behavior and a non-parametric bootstrap.

Applied to a global database of terrorist events, they estimate the worldwide historical probability of observing at least one 9/11-sized or larger event since 1968 to be 11-35 percent. “These results are robust to conditioning on global variations in economic development, domestic versus international events, the type of weapon used and a truncated history that stops at 1998,” they write. They then use this procedure to make a data-driven statistical forecast of at least one similar event over the next decade.

— Read more in Aaron Clauset and Ryan Woodard, “Estimating the historical and future probabilities of large terrorist events,” arXiv:1209.0089v1 [physics.data-an]