But criminologist James Alan Fox of Northeastern University in Boston, who studies mass killings, says the Mass Shooting Tracker totals are flawed.

“Four people with minor injuries, they count it, but three people killed, they don’t count it,” Fox said of Mass Shooting Tracker's criteria. “Everyone would agree three people killed is worse than four people with minor injuries.... It's really blending together apples with watermelons.”

A Los Angeles Times examination of Mass Shooting Tracker's data showed that in 42% of the 353 shootings the site counted in 2015, no one was killed. (Other news outlets previously reported this week that the tracker had logged 355 mass shootings, not 353; it’s not clear where the outlets got that number or whether the site downgraded its total as of Friday.)

The tracker has also only been gathering data since 2013, making it difficult to evaluate whether mass shootings are getting more or less common.