If you follow the news right after a crime like yesterday's mass murder in California, you've probably encountered the Mass Shooting Tracker. A crowdsourced project with roots in a Reddit subgroup, the tracker is the source for such headlines as "The San Bernardino shooting is America's 1,044th mass shooting in 1,066 days" (at Vox) and "The San Bernardino shooting is the second mass shooting today and the 355th this year" (at Wonkblog, the original Vox). Its numbers turn up in news outlets across the country, and those numbers always sound enormous.

This marks a sharp change from the way the issue was discussed a few years ago. In the wake of the Sandy Hook massacre, the go-to source for places like Wonkblog was Mother Jones' "Guide to Mass Shootings in America," which first appeared in 2012 and has been periodically updated since then. It currently lists 72 incidents since 1982, a rather smaller figure than the tracker provides. That's because Mother Jones uses a much narrower definition of "mass shooting," trying to focus on crimes like Columbine without roping in any robber who fires a gun at several people. But Mother Jones also lists more incidents in recent years than earlier. If the Mass Shooting Tracker alarms people by showing them a big number, the Mother Jones list alarms people by suggesting such shootings are happening more frequently.

When skeptics started challenging Mother Jones' list, they usually pointed instead to yet another source: the FBI, which has more or less defined a mass shooting as a gun crime where four or more people are murdered. (*) The FBI stats showed many more mass-shooting incidents and casualties than Mother Jones did, but they also undermined the idea that the numbers were shooting upward; these figures zig-zagged erratically for decades, sometimes rising and sometimes falling, without any discernible trend. Mother Jones argued that its approach was better because it didn't mix "mass shootings in public places with a far more numerous set of mass murders that are contextually distinct—a majority of which stem from domestic violence and occur in private homes."

The Mass Shooting Tracker, by contrast, uses an even more expansive definition than the FBI's: not incidents that leave at least four people killed but incidents in which at least four people are hit.

Obviously, this gives you a bigger total than any of the other measurements. But while the feds' stats go back years, the Shooting Tracker begins in 2013—and because its data come from press accounts rather than police records, it's a somewhat shaky source for year-to-year comparisons anyway. (This is also a problem with the Mother Jones list, which tends to undercount shootings that did not happen relatively recently. The amount of press coverage that different sorts of crime attract inevitably varies over time.) So basically, the Mass Shooting Tracker offers the biggest available estimate of recent shootings with no way to see if the crimes that fit its definition are becoming more or less common.

If you want to get a sense of both the size of the problem and the trend over time, your best bet is a Congressional Research Service study released this past summer. It looks both at mass shootings as broadly defined via the FBI and at Sandy Hook-style public shootings; for the latter category, it relies not on the Mother Jones count but on a more comprehensive dataset assembled by the criminologist Grant Duwe. (Rather than starting with a search of news accounts, Duwe begins with the FBI's homicide data to find out when and where mass killings happened, then uses the available media reports to fill in the details.) I described that paper's conclusions in detail here, but these are the highlights:

Mass shootings were slightly more common, and killed a larger number of people, in 2009-2013 than in the previous two five-year periods. The congressional study attributes both increases largely to one outlier year, noting that 2012 was unusually bad.

Few mass shootings are mass public shootings.

More people die in mass public shootings now than in previous decades, but if you adjust for population growth that increase basically disappears.

There will always be many ways to slice the data, and it's not a bad thing that there are multiple measurements out there, as long as readers understand what precisely each one describes. But the most important figures of all are the ones you get if you cast a really wide net—wider even than the Reddit group's—and look at the rates for gun violence of all kinds. In that case, the numbers are clear: Both fatal and nonfatal firearm crimes have dropped dramatically over the last two decades.

(* Strictly speaking, this is how the FBI has defined mass murder, with the added requirement that the killings be committed with a firearm. There is no official FBI definition of "mass shooting.")