How many mass shootings are there, really?

On Wednesday, a Washington Post article announced that “The San Bernardino shooting is the second mass shooting today and the 355th this year.” Vox, MSNBC’s Rachel Maddow, this newspaper and others reported similar statistics. Grim details from the church in Charleston, a college classroom in Oregon and a Planned Parenthood clinic in Colorado are still fresh, but you could be forgiven for wondering how you missed more than 300 other such attacks in 2015.

At Mother Jones, where I work as an editor, we have compiled an in-depth, open-source database covering more than three decades of public mass shootings. By our measure, there have been four “mass shootings” this year, including the one in San Bernardino, and at least 73 such attacks since 1982.

What explains the vastly different count? The answer is that there is no official definition for “mass shooting.” Almost all of the gun crimes behind the much larger statistic are less lethal and bear little relevance to the type of public mass murder we have just witnessed again. Including them in the same breath suggests that a 1 a.m. gang fight in a Sacramento restaurant, in which two were killed and two injured, is the same kind of event as a deranged man walking into a community college classroom and massacring nine and injuring nine others. Or that a late-night shooting on a street in Savannah, Ga., yesterday that injured three and killed one is in the same category as the madness that just played out in Southern California.

While all the victims are important, conflating those many other crimes with indiscriminate slaughter in public venues obscures our understanding of this complicated and growing problem. Everyone is desperate to know why these attacks happen and how we might stop them — and we can’t know, unless we collect and focus on useful data that filter out the noise.

For at least the past decade, the F.B.I. regarded a mass shooting as a single attack in which four or more victims were killed. (In 2013, a mandate from President Obama for further study of the problem lowered that threshold to three victims killed.) When we began compiling our database in 2012, we used that criteria of four or more killed in public attacks, but excluded mass murders that stemmed from robbery, gang violence or domestic abuse in private homes. Our goal with this relatively narrow set of parameters was to better understand the seemingly indiscriminate attacks that have increased in recent years, whether in movie theaters, elementary schools or office parks.

Read more from the NYTimes op-ed