Eleven schools since Columbine High School in 1999 have had mass shootings. Accounts by witnesses and survivors are eerily similar. (Monica Akhtar/The Washington Post)

The stunning number swept across the Internet within minutes of the news Wednesday that, yet again, another young man with another semiautomatic rifle had rampaged through a school, this time at Marjory Stoneman Douglas High in South Florida.

The figure originated with Everytown for Gun Safety, a nonprofit group, co-founded by Michael Bloomberg, that works to prevent gun violence and is most famous for its running tally of school shootings.

“This,” the organization tweeted at 4:22 p.m. Wednesday, “is the 18th school shooting in the U.S. in 2018.”

A tweet by Sen. Bernie Sanders (I-Vt.) including the claim had been liked more than 45,000 times by Thursday evening, and one from political analyst Jeff Greenfield had cracked 126,000. New York City Mayor Bill de Blasio tweeted it, too, as did performers Cher and Alexander William and actors Misha Collins and Albert Brooks. News organizations — including MSNBC, ABC News, NBC News, CBS News, Time, MSN, the BBC, the New York Daily News and HuffPost — also used the number in their coverage. By Wednesday night, the top suggested search after typing “18” into Google was “18 school shootings in 2018.”

It is a horrifying statistic. And it is wrong.

Everytown has long inflated its total by including incidents of gunfire that are not really school shootings. Take, for example, what it counted as the year’s first: On the afternoon of Jan. 3, a 31-year-old man who had parked outside a Michigan elementary school called police to say he was armed and suicidal. Several hours later, he killed himself. The school, however, had been closed for seven months. There were no teachers. There were no students.

Also listed on the organization’s site is an incident from Jan. 20, when at 1 a.m. a man was shot at a sorority event on the campus of Wake Forest University. A week later, as a basketball game was being played at a Michigan high school, someone fired several rounds from a gun in the parking lot. No one was injured, and it was past 8 p.m., well after classes had ended for the day, but Everytown still labeled it a school shooting.

Everytown explains on its website that it defines a school shooting as “any time a firearm discharges a live round inside a school building or on a school campus or grounds.”

Sarah Tofte, Everytown’s research director, calls the definition “crystal clear,” noting that “every time a gun is discharged on school grounds it shatters the sense of safety” for students, parents and the community.

She said she and her colleagues work to reiterate those parameters in their public messaging. But the organization’s tweets and Facebook posts seldom include that nuance. Just once in 2018, on Feb. 2, has the organization clearly explained its definition on Twitter. And Everytown rarely pushes its jarring totals on social media immediately after the more questionable shootings, as it does with those that are high-profile and undeniable, such as the Florida massacre or one from last month in Kentucky that left two students dead and at least 18 people injured.

After The Washington Post published this report, Everytown removed the Jan. 3 suicide outside the closed Michigan school.

The figures matter because gun-control activists use them as evidence in their fight for bans on assault weapons, stricter background checks and other legislation. Gun rights groups seize on the faults in the data to undermine those arguments and, similarly, present skewed figures of their own.

Gun violence is a crisis in the United States, especially for children, and a huge number — one that needs no exaggeration — have been affected by school shootings. An ongoing Washington Post analysis has found that more than 150,000 students attending at least 170 primary or secondary schools have experienced a shooting on campus since the Columbine High School massacre in 1999. That figure, which comes from a review of online archives, state and federal enrollment figures and news stories, is a conservative calculation and does not include dozens of suicides, accidents and after-school assaults that have also exposed youths to gunfire.

Just five of Everytown’s 18 school shootings listed for 2018 happened during school hours and resulted in any physical injury. Three others appeared to be intentional shootings but did not hurt anyone. Two more involved guns — one carried by a school police officer and the other by a licensed peace officer who ran a college club — that were unintentionally fired and, again, led to no injuries. At least seven of Everytown’s 18 shootings took place outside normal school hours.

Shootings of any kind, of course, can be traumatic, regardless of whether they cause physical harm.

A month ago, for example, a group of college students were at a meeting of a criminal-justice club in Texas when a student accidentally fired a real gun, rather than a training weapon. The bullet went through a wall, then a window. Though no one was hurt, it left the student distraught.

Is that a school shooting, though? Yes, Everytown says.

“Since 2013,” the organization says on its website, “there have been nearly 300 school shootings in America — an average of about one a week.”

But since Everytown began its tracking, it has included these dubious examples — in August 2013, a man shot on a Tennessee high school’s property at 2 a.m.; in December 2014, a man shot in his car late one night and discovered the next day in a Pennsylvania elementary school’s parking lot; in August 2015, a man who climbed atop the roof of an empty Texas school on a Sunday morning and fired sporadically; in January 2016, a man in an Indiana high school parking lot whose gun accidentally went off in his glove box, before any students had arrived on campus; in December 2017, two teens in Washington state who shot up a high school just before midnight on New Year’s Eve, when the building was otherwise empty.

In 2015, The Post’s Fact Checker awarded the group’s figures — invoked by Sen. Chris Murphy (D-Conn.) — four Pinocchios for misleading methodology.

Another database, the Gun Violence Archive, defines school shootings in much narrower terms, considering only those that take place during school hours or extracurricular activities.

Yet many journalists rely on Everytown’s data. Post media critic Erik Wemple included the 18 figure in a column Wednesday night, and Michael Barbaro, host of the New York Times’ podcast “The Daily,” used the number to punctuate the end of his Thursday show.

Much like trying to define a mass shooting, deciding what is and is not a school shooting can be difficult. Some obviously fit the common-sense definition: Last month, a teen in Texas opened fire in a school cafeteria, injuring a 15-year-old girl.

Others that Everytown includes on its list, though, are trickier to categorize.

About 6 p.m. Jan. 10, a bullet probably fired from off campus hit the window of a building at a college in Southern California. No one was hurt, but students could still have been frightened. Classes were canceled, rooms were locked down and police searched campus for the gunman, who was never found.

On Feb. 5, a police officer was sitting on a bench in a Minnesota school gym when a third-grader accidentally pulled the trigger of his holstered pistol, firing a round into the floor. None of the four students in the gym were injured, but, again, the incident was probably scary.

What is not in dispute is gun violence’s pervasiveness and its devastating impact on children. A recent study of World Health Organization data published in the American Journal of Medicine that found that, among high-income nations, 91 percent of children younger than 15 who were killed by bullets lived in the United States.

And the trends are only growing more dire.

On average, two dozen children are shot every day in the United States, and in 2016 more youths were killed by gunfire — 1,637 — than during any previous year this millennium.