Calculating US police killings using methodologies from war-crimes trials

Statistician Patrick Ball runs an NGO called the Human Rights Data Analysis Group, which uses extremely rigorous, well-documented statistical techniques to provide evidence of war crimes and genocides; HRDAG's work has been used in the official investigations of atrocities in Kosovo, Guatemala, Peru, Colombia, Syria and elsewhere.

HRDAG is called upon to estimate the full scope of killings by soldiers and police in situations where no records have been kept, or when the records have systematically excluded the worst offenses, and where witnesses and survivors fear reprisals and will not come forward. Through their decades of experience, Ball and HRDAG have developed methodologies for producing good estimates even where the data is in such disarray.

One place where this disarray exists is in the USA, where no comprehensive records are kept of police killings, where some jurisdictions openly refuse to supply any statistics on such killings, and where fear of reprisals and an unjust system prevent many incidents from being reported at all.

In a must-read article in Granta>, Ball explains the fundamentals of statistical estimation, and then applies these techniques to US police killings, merging data-sets from the police and the press to arrive at an estimate of the knowable US police homicides (about 1,250/year) and the true total (about 1,500/year).

That means that of all the killings by strangers in the USA, one third are committed by the police.

This is data-driven journalism at its finest: uncovering socially vital data that has been suppressed at the highest levels, while providing an education in how to estimate numbers like these from fragmentary data-sources.

To understand the impact of the correlation between one list organized by the police – like the Supplementary Homicide Report – and another list organized from media sources – like the Arrest-Related Deaths database – it's most useful to compare them to other cases where we have similar kinds of lists, that is, police and media lists. And the range of correlations that are most informative for our investigation are those in Colombia, where there is a very effective police reporting database, and good databases maintained by human rights groups of homicides reported in the press.

Using the correlations from these lists, we conclude that for the eight-year period included in the study by the Bureau of Justice Statistics, it is likely that there were approximately 10,000 homicides committed by the police, that is, about 1,250 per year. Keep in mind that the Bureau of Justice Statistics report itself excludes many jurisdictions in the United States that openly refuse to share any data with the FBI. The true number of homicides committed by police is therefore even higher. Though not a true estimate, my best guess of the number of police homicides in the United States is about 1,500 per year.

As I said at the beginning of this article, the estimate of 1,500 police homicides per year would mean that eight to ten per cent of all American homicide victims are killed by the police. Of all American homicide victims killed by people they don't know, approximately one-third of them are victims of the police.

Violence in Blue [Patrick Ball/Granta]