It’s not by chance that six cyclists were killed in the capital in 14 days in November, say statisticians

‘Black November’ of 2013, when six London cyclists were killed in the space of 14 days, was a sign of increased danger on the capital’s roads, a new study has found.

Published in Significance, the Journal of the Royal Statistics Society, the statisticians Jody Aberdein and David Spiegelhalter looked at the deaths of six cyclists who were killed while cycling in London.

Their piece poses the question: “as an urban cyclist, should you hang up your hat and fluoro? Should you forsake two wheels for four, or more, in order to preserve life and limb?

“Many will have started their journeys to work this month with more trepidation that they did in November.”

They then attempt to answer the question by considering how unusual six sequential deaths might be.

As they note: ‘Randomness is clumpy, and humans are excellent at pattern recognition.

“The issue with rare events which are subject to chance is that it is very hard as an observer to know whether there is an underlying pattern.

“We have, for a variety of reasons, a great propensity to attribute cause to chance occurrence.

“This tendency is never more active than when there is an emotive and high-stakes outcome.”

However the pair go on to study data from road traffic accidents for the whole of the UK for the last several years, concluding that there are historically on average 0.6 cyclist deaths per fortnight over the period 2005 to 2012.

Even plotting the deaths out over calendar months, in 8 years the statisticians find only five fortnights with 3 deaths in them.

Aberdein and Spiegelhalter go on to conclude that “It turns out that over 8 years the chance of our seeing six deaths in a 14-day period is around 2.5%.

“The usual level of “significance” in such analyses is set, arbitrarily, at 5%. In other words, the chance of observing an occurrence at least as extreme under the baseline assumption is less than 5%.

“Hence six deaths is a significant finding.”

After an unpromising start, having to be bribed by her parents to learn to ride without stabilisers, Sarah became rather keener on cycling in her university years, and was eventually persuaded to upgrade to proper road cycling by the prospect of a shiny red Italian bike, which she promptly destroyed by trapping a pair of knickers in the rear derailleur. Sarah writes about about cycling every weekend on road.cc.