The Frequency of Terrorist Attacks

Dr. Gordon Woo, Risk Management Solutions is a catastrophist, specializing in the probabilistic risk assessment of catastrophe insurance risks. Since 9/11, he has pioneered methods for quantifying terrorism risk. In 2004, he was named by Treasury & Risk magazine as one of the hundred most influential people in finance.

The Frequency of Terrorist Attacks

The US Secretary of Homeland Security, Michael Cherthoff has been in the public eye this summer for his handling of the Katrina and Rita hurricane crises, rather than for terrorism. New Orleans was still a flooded ghost town when the fourth anniversary of 9/11 came and went.

Why has there not been a successful terrorist attack on the US mainland within the past four years? The answer is not that the terrorists are giving the US a break; rather, the counter-terrorism and law enforcement services have been doing a very good job in interdicting planned attacks.

Hurricane formation and motion are determined by meteorological factors beyond human intervention. By contrast, terrorist activity is subject to counter-terrorism control. With hurricanes, it's not a question of when one might occur, but how often; with terrorist campaigns, it's not a question of if, but when.

A single successful terrorist attack in the USA is a major psychological blow, and a harsh indictment of government security policy. Post 9/11, stringent measures such as the Patriot Act and the US Visit program have been introduced to control the Al Qaeda threat. As a result, out of about ten planned attacks on the US mainland since 9/11, none has been successful. The interdiction rate of planned attacks in the USA has been admirably high.

Prof. Paul Wilkinson, the esteemed chairman of the St. Andrews Centre for the Study of Terrorism and Political Violence, has used a football metaphor to explain the confrontation between the opposing forces of terrorism and counter-terrorism: ‘Fighting terrorism is like being a goalkeeper. You can make a hundred brilliant saves, but the only shot that people remember is the one that gets past you.' Good goalkeepers can save about four shots out of five. This is similar to the typical interdiction rate of high tempo professional counter-terrorist operations, whether in Ireland or Israel, and seems to reflect experience post 9/11 in both USA and UK.

Major Al Qaeda attack plans are meticulous in the detail of reconnaissance, surveillance and rehearsal, and take a substantial amount of logistical resources and time to prepare. Accordingly, relatively few attacks can be mounted each year against the US mainland. Given the high likelihood of such attacks being thwarted, the end result is that the annual frequency of successful terrorist strikes is being controlled to a low level.

Secretary Cherthoff should have some time to keep an eye on the hurricane season each year.