Every time a major school shooting occurs in the United States, the national conversation that immediately follows recounts the number of similar incidents that have happened to date that year. The chilling statistics, broadcast over the airwaves, published in newsprint and shared on the internet, vary depending on the news organization and its definition of a school shooting.

Several organizations and databases track gun violence in schools, including Everytown for Gun Safety, the Washington Post school shooting database, and the U.S. Department of Education. These publishers contribute to the public’s understanding of the effect of gun violence even though each might provide different numbers for school shootings that have occurred in a particular time period.

Dr. Daniel Webster, a professor at the Johns Hopkins Center for Gun Policy and Research, said that part of the difficulty in studying gun violence is determining what counts as a school shooting.

“Let’s say someone is shot on school grounds in the evening,” Webster said. “It has nothing to do with the school day and doesn’t involve a student, but you could identify that in a database as the setting is a school. That makes things murky.”

The differing totals can lead to confusion about the number of school shootings. In the hours after the mass shooting at the Marjory Stoneman Douglas High School in Parkland, Florida on Feb. 14 2018, Everytown for Gun Safety, which tracks every instance of gunfire on school grounds, tweeted that it was the 18th such event in 2018. The tweet was retweeted more than 800 times, prompting the Washington Post to note that only five of the 18 instances occurred during school hours and resulted in injury.

Dr. Lacey Wallace, Penn State University assistant professor of criminal justice, told NBC News that broad definitions create inflated shooting totals, and inflated totals lead to public fear.

“The higher that number goes, the more people are afraid to send their kids to school,” Wallace said.

“Our schools are actually pretty safe, and in a lot of cases they’re safer than the surrounding communities,” Wallace said.

In an effort to be more consistent, and to more closely align our statistics with our reporting, teams within NBC News collaborated, studied existing school shooting databases and sources, and created a criteria for planned school shooting counts for use across the company’s newsrooms.

The federal Safe School Initiative was formed after the 1999 Columbine High School massacre to help determine how to prevent future attacks. Its report looks at “incidents of ‘targeted violence’ in school settings… where the school was deliberately selected.”

In an effort to capture the terror of an active shooter entering a school, the NBC News shooting tracker criteria focuses on the type of incidents described in the Safe School Initiative report.

The NBC News school shooting tracker includes shootings that meet these criteria:

1. One or more active shooters. The FBI defines an active shooter as an individual engaged in attempting to kill people in a confined space or populated area.

2. On school property during school hours and as students are arriving or leaving, or at school-sanctioned or school-sponsored events. “Schools” are defined as ranging from nursery schools to colleges, universities, and technical schools.

3. There is intent to harm students or faculty with a gun.

4. At least one person, other than the shooter, is injured or dies.

And while every case of school gun violence is serious and can cause trauma and suffering for the children and adults who experience it, in order to capture the subset of gun violence described in the Safe School Initiative report our count excludes the following cases:

1. Accidental discharge of a weapon at school

2. Suicide by firearm at school

3. Isolated fights, altercations, or domestic disputes*, including gang violence

* There may be exceptions in which a domestic dispute crosses over into the realm of a planned school shooting attack.

Our data is derived from an analysis of information from news reports, social media monitoring, the K-12 School Shooting Database, Everytown for Gun Safety, law enforcement and government reports and press conferences, court records, our own reporting, and other publicly available information. Shooting events are recorded and evaluated as new information becomes available and are added to our published dataset when it’s determined an incident meets the NBC News standard for planned school shootings carried out by an active shooter.