By Mark Follman, Los Angeles Times (MCT)

It’s not a matter of if, but when and where the next mass shooting will happen: It might take place at another shopping mall, or college campus, or suburban office building, and probably not long from now. Yet, as these disturbing incidents keep appearing in the headlines, various commentators have argued that mass shootings are not on the rise.

That’s true if you look at all mass shootings, including gang killings and in-home violence stemming from domestic abuse. But new research from the Harvard School of Public Health demonstrates that mass shootings in public have become far more frequent. The Harvard findings are also corroborated by a separate report issued recently by the FBI.

After a heavily armed young man gunned down 12 people and wounded 58 others at a movie theater in Aurora, Colorado, in July 2012, my colleagues at Mother Jones and I began examining how often mass shootings in public places occurred. Finding no reliable answer, we set about gathering three decades of data. We discovered that such shootings were on the rise — even before the horror at Sandy Hook Elementary, the Washington Navy Yard, Ft. Hood and near UC Santa Barbara.

Though mass shootings make an outsized psychological impact, they are a tiny fraction of the nation’s overall gun violence, which takes more than 30,000 lives annually. Rather than simply tallying the yearly number of mass shootings, Harvard researchers Amy Cohen, Deborah Azrael and Matthew Miller determined that their frequency is best measured by tracking the time between each incident. This method, they explain, is most effective for detecting meaningful shifts in relatively small sets of data, such as the 69 mass shootings we documented. Their analysis of the data shows that from 1982 to 2011, mass shootings occurred every 200 days on average. Since late 2011, they found, mass shootings have occurred at triple that rate — every 64 days on average.

There has never been a clear, universally accepted definition of “mass shooting.” Our data include attacks in public places with four or more victims killed, a baseline established by the FBI a decade ago. We excluded mass murders in private homes stemming from domestic violence, as well as shootings tied to gang or other criminal activity. (Qualitative consistency is crucial, even though any definition can at times seem arbitrary. For example, by the four-fatalities threshold neither the attack at Ft. Hood in April nor the one in Santa Barbara in May qualifies as a “mass shooting,” with three victims killed by gunshots in each incident.) A report from the FBI on gun rampages, issued in late September, includes attacks with fewer than four fatalities but otherwise uses very similar criteria.

The FBI report, which includes 160 “active shooter” cases between 2000 and 2013, notes explicitly that it is not a study of mass shootings. Rather, it analyzes incidents in which shooters are “actively engaged in killing or attempting to kill people” in a public place, regardless of the number of casualties. But within the FBI’s 160 cases is a subset of 44 mass shootings (in which four or more were murdered) nearly identical to Mother Jones’ data set for the same period. The Harvard researchers underscore that the FBI had access to law enforcement sources that Mother Jones did not: “That the results of the two studies are so similar reinforces our finding that public mass shootings have increased.”

James Alan Fox, a widely quoted researcher from Northeastern University in Boston, has argued that mass shootings are not on the rise, and that they are too rare to merit significant policy changes. As he put it recently in an interview with CNN’s Jake Tapper: “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.” But in drawing his conclusions, Fox examined data for all mass shootings, including those that stemmed from domestic violence and those tied to criminal activity. Such killings are no less awful, 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.

The question of whether mass shootings in public can be prevented hinges on understanding the complex factors behind them — which starts with tracking these shootings accurately. That, at least, is a role that the federal government is poised to assume: Last year President Obama signed the Investigative Assistance for Violent Crimes Act, which authorizes the Department of Justice to investigate mass shootings in public places. Notably, the law defines the threshold for mass shootings as three or more people murdered — which means eventually we’ll have data showing that the scope of the problem is far greater than we’ve already seen.

Mark Follman is a senior editor at Mother Jones, where he leads the news outlet’s award-winning coverage of gun violence in America. He wrote this for the Los Angeles Times.

Photo: brian.ch via Flickr

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