Family members embrace following a shooting at Marjory Stoneman Douglas High School in Parkland, Fla. on Wednesday.Wilfredo Lee / AP

A high-school shooting in Parkland, Florida, on Wednesday afternoon has left 17 people dead and numerous others injured, according to officials. The suspect, a 19-year-old former student named Nikolas Cruz who was expelled for unspecified disciplinary reasons, is in custody, according to officials.

As details of the attack emerge, news organizations are engaging in a grim tradition: tallying the massacre on an ever-growing list of school shootings in the U.S., and of mass shootings more generally. The Daily Beast, for example, on Wednesday cited data from Everytown, the gun-control advocacy group, which called it the 59th shooting at or near schools this academic year.

But the counting of school shootings, and of other types of shootings and incidents of mass violence, isn’t a straightforward process. Even The Daily Beast’s reference raises the question of whether reporting ought to distinguish between shootings near schools and those in schools. Meanwhile, many oft-cited statistics disregard forms of school violence that may not have involved guns but are similar to shootings in intention or impact. The messiness of counting school shootings often contributes to sensationalizing or oversimplifying a modern trend of mass violence in America that is seemingly becoming more entrenched. Wednesday’s news out of South Florida has already spurred renewed calls for policy changes and other forms of action to prevent future shootings—but such initiatives are informed by the information that’s available, and currently, that information is confusing at best and inaccurate at worst.  

Even the federal government defines and counts mass shootings in inconsistent and murky ways. Starting in 2008, the FBI limited its definition of mass shootings to a single incident in which a shooter kills four or more people, according to the criminologist Frederic Lemieux, writing in the Chicago Tribune. But in 2013 the agency decided to rely on a definition for an “active shooter” instead of narrowing in on a definition of “mass shootings”; it defined an “active shooter” as a person “actively engaged in killing or attempting to kill people in a confined and populated area.” This “active shooter” definition includes incidents in which fewer than four people die. The definition change makes historical study of the issue especially complicated given the variation in what counted as a mass shooting before 2008 and what counts now. Further complicating matters is that, after the 2012 shootings in Newtown, Connecticut, Congress officially defined “mass killings” as three or more killings in a single incident. Twenty schoolchildren were among those murdered in that attack.

Definitions are further blurred in journalism and other forms of public discourse. Inconsistent data can compel whomever’s analyzing it to falsely equate school shootings and other kinds of violence, when in reality the nuances of motivation and context might be different. A simple, seemingly straightforward number can grab attention, but exaggerated figures could also risk contributing to the public sense of numbness to shootings that already exists. That numbness poses its own danger—how can the public combat gun violence that it doesn’t truly confront? Separating out different forms of gun violence in the statistics and reporting is crucial in better understanding why shootings happen and how they might be prevented.

Coming up with consistent definitions for accurately tracking these incidents can help identify trends and influence policy proposals. In 2015, the Northwestern management-school professor Adam Pah read a headline with a count of the year’s school shootings and realized that he couldn’t objectively fact-check it. Instead, he found lots of contradicting statistics. So Pah set out to create what would become one of the most comprehensive existing databases on school shootings between 1990 and 2013, using three criteria: The incident must involve a gun being discharged, it must involve students or school staff in some way, and it must take place on a school’s premises. He ended up finding a correlation between economic distress and school shootings—the rate of shootings rose and fell in conjunction with that of economic factors such as unemployment and foreclosure.

The lack of reliable information on school shootings and other gun-related mass violence isn’t just a matter of inconsistency in definitions; political factors have also played a role in limiting access to information. Under pressure from the National Rifle Association, Congress in 1996 prohibited the Centers for Disease Control and Prevention from funding public-health research on issues related to firearms. These prohibitions have largely persisted, and there is still no comprehensive federal database on gun deaths, let alone on school shootings.

Still, there’s been progress in the quest for better data. After the deadly Aurora, Colorado, movie-theater shooting, Mother Jones created the first open-source database of mass shootings in the U.S. And the newspaper Education Week last month launched a database that counts school shootings. Each database has its own criteria for what it defines as a mass or school shooting. Mother Jones tallied “indiscriminate rampages in public places resulting in four or more victims killed by the attacker,” excluding other gun crimes like armed robbery or gang violence (a decision that was criticized by some as a narrow approach). EdWeek, meanwhile, is tracking “shootings in K-12 schools this year that involve a firearm and result in at least one shooting-related injury or death.”

Some members of Congress have publicly lamented Wednesday’s shooting, emphasizing that it’s just the latest example of why political action on gun reform is so critical. Connecticut Democratic Senator Chris Murphy, who was in office at the time of the 2012 mass shooting at Sandy Hook elementary school, said to Congress in a floor speech Wednesday: “We are responsible for a level of mass atrocity that happens in this country with zero parallel anywhere else. As a parent, it scares me to death that this body doesn't take seriously the safety of my children, and it seems like a lot of parents in South Florida are going to be asking that same question later today.”