Fatal shootings by police are the rare outcomes of the millions of encounters between police officers and the public. Despite the unpredictable events that lead to the shootings, in each of the past four years police nationwide have shot and killed almost the same number of people – nearly 1,000.
Last year police shot and killed 998 people, 11 more than the 987 they fatally shot in 2017. In 2016, police killed 963 people, and 995 in 2015.
Years of controversial police shootings, protests, heightened public awareness, local police reforms and increased officer training have had little effect on the annual total. Everyone agrees – criminal justice researchers, academics and statisticians – that all of the attention has not been enough to move the number.
Mathematicians, however, say that probability theory may offer one explanation. The theory holds that the quantity of rare events in huge populations tends to remain stable absent major societal changes, such as a fundamental shift in police culture or extreme restrictions on gun ownership, which are unlikely.
“Just as vast numbers of randomly moving molecules, when put together, produce completely predictable behavior in a gas, so do vast numbers of human possibilities, each totally unpredictable in itself, when aggregated, produce an amazing predictability,” said Sir David Spiegelhalter, a professor and statistician at the University of Cambridge who studies risk and uncertainty.