Just how many mass shootings have there been?

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.

The statistics now being highlighted in the news come primarily from shootingtracker.com, a website built by members of a Reddit forum supporting gun control called GunsAreCool. That site aggregates news stories about shooting incidents — of any kind — in which four or more people are reported to have been either injured or killed.

It’s not clear why the Redditors use this much broader criteria. The founder of the “shooting tracker” project, who currently goes by the handle “Billy Speed,” told me it was his choice: “Three years ago I decided, all by myself, to change the United States’ definition of mass shooting.” It’s also not clear how many of those stories — many of them from local outlets, including scant detail — are accurate.