Phillip Ball in Nature:
Computer scientists Aaron Clauset and Maxwell Young of the University of New Mexico, Albuquerque, have analysed the data on terrorist attacks compiled by the National Memorial Institute for the Prevention of Terrorism in Oklahoma City. They say the numbers follow a ‘power-law’ relationship.
A graph of the number of attacks n plotted against their severity x (in terms of injuries and/or fatalities) reveals that n is roughly proportional to x -1.85. Put simply, this means that the frequency of attacks decreases as their size increases – which is what you’d expect – but also that this relationship holds for events ranging from those that injured or killed just a few people to those that, like the Nairobi car bomb in August 1998, produced over 5000 casualties.
This might sound like no more than a formal way of presenting the statistics, but the power-law relationship has startling implications. For example, Clauset and Young say that the statistics suggest a strong probability of an attack as devastating as that on the World Trade Center within seven years.
More here.