(NPR)- In the United States, a black person is about three times as likely to be killed by police as a white person. That’s according to the Mapping Police Violence project. Official data on the shootings isn’t available for many jurisdictions. There are lots of theories about the reasons behind police killings, including new data linking police unionization with police violence. Stacey Vanek Smith and Cardiff Garcia from our daily economics podcast The Indicator From Planet Money report on consequences of police unionization, particularly for black Americans.