According to Granta - Violence in Blue,
One-third of all Americans killed by strangers are killed by police.
Is this true?
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To a reasonable approximation this appears to be true, provided we restrict ourselves to deliberate killings, although one quarter may be a more accurate figure.
The total number of murders in the US for 2018 is 15,498, according to the FBI, and this roughly agrees with data from other sources.
The US Department of Justice says that approximately 80% of murder victims know their killers, meaning that around 3100 people were murdered by strangers.
Recent statistics for police killings are harder to come by, but various sources indicate a rate of around 1100 for the years 2015 and 2016, and no indication that this would have changed significantly in 2018. Statista gives 967 people shot to death by police in 2018, but excludes those killed in other ways (George Floyd would not be included). It includes all killings, not just murders.
Taking 1100 police killings and 3100 people murdered by strangers, then we can conclude that the number of people killed by the police is about one third of the number of people murdered by strangers. We are neglecting here any people killed by strangers that are not murders, such as gun or vehicle accidents or self-defense killings. This is not exactly the claim being made, but it is probably where the figure comes from.
A better calculation would be to consider not just the number of stranger murders but also the number of justifiable homicides (e.g. self-defense killings). The police killings already include justifiable homicides, and there are around 300 justifiable homicides by private citizens a year giving us about 4600 deliberate killings by strangers, of which just under one quarter were killed by police.
Note that most police killings are considered justified, which means that far less than a quarter of people murdered by strangers were murdered by police.
For comparison the same calculation for the UK gives a figure of about 1 in 200.
this appears to be false as written. police only kill about 1k people per year, whereas car crashes kill somewhere around 30k people a year in the usa. 53% of car crashes involve more than 1 vehicle. if we assume that half of the car crashes victims in those are "strangers" to the driver who caused the crash. that would put the ratio around 8:1 without even counting other causes of death (e.g. crime). if we were to rephrase the claim as "one third of people who are shot to death by strangers are shot to death by the police", then that might be close to the truth.