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.