Black Lives Matter (BLM) began when Oakland, Calif.–based activist Alicia Garza posted a message of protest on Facebook after George Zimmerman, a neighborhood-watch volunteer who followed and fatally shot 17-year-old Trayvon Martin in Sanford, Fla., was acquitted of murder in 2013. “The question becomes, ‘Are Black Lives Matter protests having any real effect in terms of generating change?’ The data show very clearly that where you had Black Lives Matter protests, killing of people by the police decreased. The new study compared police killings in cities that experienced BLM protests with those that did not. “It’s really difficult to measure Black Lives Matter protests and lethal use of force by the police,” notes the study’s author, Travis Campbell, an economist at the University of Massachusetts Amherst. Aislinn Pulley is a co-founder of Black Lives Matter Chicago, which has compiled a list of everyone who died at the hands of the Chicago Police Department since 2011.