FILE - In this July 23, 2012 file photo, James E. Holmes appears in Arapahoe County District Court in Centennial, Colo. Holmes is charged with 166 counts, mostly murder and attempted murder, in the July 20 assault on moviegoers at a midnight showing of "The Dark Knight Rises" in Aurora, a Denver suburb. News organizations won't be allowed to have their own cameras in the courtroom during the Colorado theater shooting trial, but they can use video and still images from a closed-circuit TV camera already in the courtroom, the judge said Tuesday, Sept. 30, 2014. (AP Photo/Denver Post, RJ Sangosti, Pool, File)

Harvard University researchers say U.S. mass shootings have surged in the last three years, contradicting earlier studies.

The Harvard researchers said the rate of mass shootings has increased threefold since 2011, occurring on average every 64 days, compared with an average of every 200 days in the years from 1982 to 2011.

The researchers used a database created by Mother Jones to look at mass shootings, which they defined as attacks that "took place in public, in which the shooter and the victims generally were unrelated and unknown to each other, and in which the shooter murdered four or more people."

The Harvard study echoes an FBI report last month that found active shooter incidents have risen dramatically in the last six years, with an average of 16.4 such shootings a year from 2007 to 2013, compared with an average of 6.4 shootings annually from 2000 to 2006.

The graph below shows how the Harvard team plotted the increase in the rate of mass shootings since 2011:

Some earlier mediareports asserted that the rate of mass killings has not gone up. That claim, the Harvard researchers said, is based on what they call flawed research by Northeastern University criminologist James Alan Fox.

Fox's research uses a broader definition of mass shootings that includes any killing with a gun in which four or more people were murdered. That includes domestic shootings that take place in a private home, and gang-related or drug-related killings.

Mark Follman, Mother Jones' senior editor, explained why the distinction in the definitions is important.

"Our focus has been on public attacks as opposed to domestic attacks," Follman told The Huffington Post. "Domestic killings are no less serious, but they are a different kind of problem. It’s terrible when a mentally disturbed person slaughters his whole family in his home, but that’s different from indiscriminate killing."

Follman did not take part in the Harvard analysis, but Mother Jones provided the data upon which the findings are based.

The researchers looked at the amount of time between mass shootings. "This is more effective than counting the annual number of incidents because it is more sensitive to detecting changes in frequency when the number of events per year is small, as is the case with public mass shootings," the researchers explained.

Follman said he hopes the data will give policymakers a clearer idea of how frequently mass shootings occur and help them decide what can be done to curb them.

"The universal goal is we want to prevent these from happening," Follman said. "You have to understand the problem better and track the problem accurately. The clearer the data we can get on mass shootings the better."

UPDATE:Criminologist Fox emailed HuffPost with a response to the Harvard research:

"Why would it be 'flawed' to include all cases of mass shootings, rather than trying to impose somewhat arbitrary criteria. Four or more killed with a gun IS a mass shooting, no mater where it takes place and by whom."