Using this power law relationship — called “scale invariance” — the risk of a large attack can be estimated by studying the frequency of small attacks. It’s a calculation that turns the usual thinking about terrorism on its head. “The conventional viewpoint has been there is ‘little terrorism’ and ‘big terrorism,’ and little terrorism doesn’t tell you anything about big terrorism,” Clauset explains. “The power law says that’s not true.”
Massive acts of violence, like 9/11 or the devastating 1995 bombing of the U.S. embassy in Nairobi, obey the same statistical rules as a small-scale IED attack that kills no one, Clauset’s work suggests. “The power law form gives you a very simple extrapolation rule for statistically connecting the two,” he says.
Although the U.S. and European nations have remained for years in a semipermanent state of high alert, the majority of terrorist attacks actually occur in the developing world, the data show — yet they are not the most severe. “Terrorist attacks happen less often in the developed world, but when they do happen, they’re often bigger than in the developing world,” Clauset says. “That was striking. We have no explanation for why that was the case.”
The size of terrorist groups is also an important variable. “The bigger they are, the faster they attack,” he says. “Most groups probably start small. They attack, they gain notoriety, they get some recruits and they get bigger. Once they reach a certain size, they can last longer.”
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