London Murders: a Predictable Pattern?

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London murders: a predictable pattern?
Four stabbings to death in a single day. Ninety murders in 7 months. Shocking figures—or are they? Knife crime makes the headlines almost daily but are Londoners really at increased risk of being murdered? David Spiegelhalter and Arthur Barnett investigate— and find a predictable pattern of murder. Violence in London attracts headlines. After four people were murdered in separate incidents in London on July 10th, 2008, BBC correspondent Andy Tighe said “To have four fatal stabbings in one day could be a statistical freak”1. But could it? And on July 28th thelondonpaper had the front page headline: “London’s murder count reaches 90”. Of course every single murder is a tragedy for those whom it affects, and it is little comfort to them what patterns we may detect in the figures. But for the benefit of the police service and broader society we may wish to consider whether these “shocking” numbers in the media are evidence that things are really getting worse. Each year the London Metropolitan Police record around 170 homicides, and this has been stable for the last 5 years2. Box 1 explains what is covered by “homicide”—here we shall use “murder” as an equivalent term. We shall focus on all murders, whatever the method of killing: Box 2 provides details about the cause of death and briefly considers stabbings alone. Each of these murders is an individual crime that cannot be predicted. It may appear strange, but this very randomness means that the overall pattern of murders is, in some ways, quite predictable. Using some basic probability theory, and assuming that the level of violence remains the same, we can answer the questions coming out of the two stories given above: first, four murders in a day is unusual but not extraordinary. We would expect it to occur around once every 3 years. And, second, we would have expected, on statistical grounds, 93 murders by July 28th—so the count of 90 is not at all surprising. We can also answer questions such as how often would we expect days with no murders? (The answer is: on around 64% of days.) How often could we expect three or more murders in one day? (On around 4 days a year.) And how often should we expect there to be long gaps between murders? (A gap of 7 days should occur around six times a year.) Why should anyone care about this? By making full use of the kind of data collected by the Home Office Specialist Crime & Police Resources Team, both the public and police forces can be alert to real changes in the incidence of crime, but not be either unnecessarily alarmed or falsely reassured by apparent variability. This analysis is part of our contribution to the work of the Risk and Regulation Advisory Council (http:// rrac.intelligus.net) in trying to stimulate public interest in a debate about “risk” in policing following on from Sir Ronnie Flanagan’s review of policing3.

Box 1. What counts as a “homicide”?
The terminology used in this article reflects reporting by the popular press rather than Home Office statistical definitions and nomenclature2. Home Office statistics report “homicides” not “murders”, and homicide by “sharp instrument” not “stabbings”. The term “homicide” covers the offences of murder, manslaughter and infanticide. Murder and manslaughter are common law offences that have never been defined by statute, although they have been modified by statute. The offence of infanticide was created by the Infanticide Act 1922 and refined by the Infanticide Act 1938 (s1). Homicide offences are shown according to the year in which the police initially recorded the offence as homicide. This is not necessarily the year in which the incident took place nor the year in which any court decision was made.

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Box 2. Four stabbings in a day?
The main body of this article addresses the question “How surprising is four murders in one day in London?” A full answer to Andrew Tighe’s question of whether four stabbings can be considered a...
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