Since 2011, mass shootings in the United States have occurred at a rate three times higher than in the previous three decades, according to a new statistical analysis from researchers at the Harvard School of Public Health.

The analysis shows that between 1982 and 2011, mass shootings occurred in the U.S. every 200 days on average. Since late 2011, that rate has tripled, to every 64 days on average.

Those findings strongly suggest, say the Harvard researchers, that “mass shootings, as of September 2011, are now part of a new, accelerated process.”

The analysis was published last week in Mother Jones magazine. It relies on data compiled by Mother Jones on mass shootings reported in the media. To be included in that database, the mass shooting had to meet three criteria: 1) the shooting had to take place in public, 2) the shooter and the victims had to be generally unrelated and unknown to each other, and 3) four or more people had to have been murdered by the shooter. (In 2013, a new federal law redefined the threshold count for mass killings to three or more from the older threshold of four or more established by the FBI a decade ago.)

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The database includes the Accent Signage shooting that occurred in Minneapolis in September 2012.

The Mother Jones database does not include other types of homicides in which four or more people were killed with a gun, such as gang killings and mass murders that are the result of domestic violence and take place in private homes.

Analyzing ‘different monsters’

As the Harvard researchers point out, past reports in the media that claim mass shootings have not increased are based primarily on the work of Northeastern University criminologist James Alan Fox, whose studies lump all types of homicides with four or more victims together.

That’s a “misguided approach to studying the problem,” say the Harvard researchers.

“Such killings are no less awful,” explains Mark Follman, a senior editor at Mother Jones, in an op-ed published Sunday in the L.A. Times, “but they are a different monster in terms of impact on public safety and the complicated policy questions they raise — not least how they might be stopped.”

Fox, however, has repeatedly stated that because his research shows that mass shootings are not increasing in frequency, no significant policy changes are needed.

“We treasure our personal freedoms in America, and unfortunately, occasional mass shootings — as horrific as they are — is one of the prices that we pay for the freedoms that we enjoy,” Fox told CNN anchor Jake Tapper last June. “I don’t want to minimize the pain and suffering of the victims and their families and those communities. They’re horrific. But it’s not an epidemic. Let’s not go in a knee-jerk way, and change the society for something that happens very rarely.”

A more effective approach

Critics of the Mother Jones database have said that it cannot be used to make any reliable conclusions about changes in the rate of mass shootings because its number of data points (the number of shootings) is too small. The Harvard researchers reply by pointing out that their method (called statistical process control) of analyzing the time interval between each shooting incident is actually “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.”

Such a method was “first developed for industry to identify changes in the process underlying a specific problem, so that root causes of that problem could be better assessed,” they explain. “This approach has proved effective in health care, for example, helping to reduce surgical errors. For the method to work, it is crucial to analyze events that are qualitatively similar. In other words, to assess the rate of public mass shootings it is necessary to exclude mass killings that are qualitatively distinct, like those taking place in private homes.”

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Findings echo those of FBI

Although Mother Jones’ method for collecting data differs somewhat from that used by the FBI, their data is remarkably similar. And so is the trend identified in their findings. In September, the FBI released a study in which it reported that the frequency of “active shooter” incidents (incidents in which shooters are “actively engaged in killing or attempting to kill people” in a public place, regardless of how many people are killed) is on the rise.

During the first seven years of the FBI study (2000-2006), an average of 6.4 “active shooter” incidents occurred annually in the U.S. During the last seven years of the study (2007-2013), the incidents had risen to an average of 16.4 per year.

“Though we now know that public mass shootings have been occurring more often, the reasons why have yet to be identified,” write the Harvard researchers. It’s unlikely, they say, that the change is due to a sudden increase within the past three years in the prevalence of mental illness.

“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,” they add.

You can read the Harvard researchers’ analysis on the Mother Jones website.