The rate of mass shootings in the United States has tripled since 2011, according to a new analysis by researchers from Harvard School of Public Health and Northeastern University. In the last three years, there have been 14 mass shootings—defined as public attacks in which the shooter and victims were generally unknown to each other and four or more people were killed—occurring on average every 64 days. During the previous 29 years, mass shootings occurred on average every 200 days.

Authors Amy Cohen, Deoborah Azrael, and Northeastern’s Matthew Miller, all affiliated with HSPH’s Health Policy and Management Department, conducted independent research using three decades-worth of data on mass shootings compiled by Mother Jones. The findings were published online October 15, 2014.

They write that the reason for the accelerating rate of mass shootings has not yet been identified. “As we search for answers with the common goal of diminishing mass shootings, studying them effectively remains key, not least for gauging the success of any policies aimed at reducing the frequency and toll of these events.”

Read Mother Jones article: Rate of mass shootings has tripled since 2011, Harvard research shows