You all probably know that the first line in the “executive summary”:http://www.preventwmd.gov/world_at_risk_preface/ of the December “report”:http://www.preventwmd.gov/report/ from the “Commission on the Prevention of Weapons of Mass Destruction Proliferation and Terrorism”:http://www.preventwmd.gov/ states that, in the Commission’s opinion,
bq. unless the world community acts decisively and with great urgency, it is more likely than not that a weapon of mass destruction will be used in a terrorist attack somewhere in the world by the end of 2013.
What you may not know is that, as far as I can tell, that claim does not appear again in the report. In any case, there is certainly no methodology to explain where that number came from.