How to measure the terrorism threat after the Westminster attack

Two questions have been particularly prominent in the aftermath of the attack on Westminster. First, why was the attacker – named by police as Khalid Masood – not apprehended, given that he appears to have been already known to police and intelligence services. Second, what does this attack mean for security in London and elsewhere.

The UK’s counter-terrorism architecture involves a five-tiered warning system organised around five levels of threat: low, moderate, substantial, severe and critical. It is similar, in this sense, to the colour-coded warnings of the US Homeland Security Advisory System – a system which was replaced in 2011 by the National Terrorism Advisory System. The current level remains at severe in the wake of the Westminster attack.

Under the UK’s system, the country can never be entirely free from the threat of terrorism: at best an attack is seen to be unlikely. The simplicity of this system also implies that threats such as terrorism can be accurately, or even objectively, measured – and their increase or decrease calculated and compared.

Questions of probability

Assumptions such as these are misleading, because terrorism threat assessments, like assessment of any type of threat, involve an inescapable element of judgement and interpretation. Prospective dangers have to be translated into calculations of likelihood and potential consequences in order to become threats. Decisions then have to be made about the relative importance of probability and impact in calculating the severity of any such dangers (READ MORE)