Harvard researchers studied the frequency of mass shootings

A policeman stands guard outside a movie theater in Aurora, Colo., where a man opened fire and killed 12 people.

Harvard research showed that in the years 1982 to 2011, a mass shooting occurred on average every 200 days. Since 2011, they found mass shootings have occurred more often.

Researchers defined a mass shooting as an attack that occurred in a public place in which four or more people were murdered.

Harvard researchers graphed their findings by plotting each incident over the number of days that pass between them .

Mother Jones provided data for Harvard researchers. Senior editor of Mother Jones, Mark Follman, said he hopes findings like these will give the government an idea of how often these shootings occur and eventually lead to a way to prevent them.