A statistic that claimed the shooting at Marjory Stoneman Douglas High School in Parkland, Florida, was the 18th this year is a misleading one, according to The Washington Post.

The number appears to have originated from a nonprofit gun control group called Everytown for Gun Safety. The group tweeted the statistic Wednesday, the day of the South Florida shooting.

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“Our hearts are with all those impacted by the shooting at Marjory Stoneman Douglas High School in Parkland, Florida today,” the tweet read. “This is the 18th school shooting in the U.S. in 2018.”

Still, the statistic was quickly spread, being included in tweets from Sen. Bernie Sanders, New York City Mayor Bill de Blasio and a number of news organizations, including NBC News, ABC News, CBS News and the BBCaccording to The Washington Post.

Everytown’s definition of what constitutes a school shooting is the reasoning for the number, which the publication says is inflated.

A school shooting, according to the Everytown website, is defined as "any time a firearm discharges a live round inside a school building or on a school campus or grounds."

“Everytown uses a straightforward, fair, and comprehensive definition for a school shooting,” the site said.

The organization's website lists one incident on Jan. 31 in which a man shot and killed himself in a parked car outside of an elementary school that had been closed for since June 2017. It was a shooting at a school, but neither students nor staff were there.

Another incident, at a Los Angeles middle school Feb. 1, was called an accidental shooting by police. Five people were injured.

Eight other incidents listed fall under the organization’s “Gun fired but no one injured” category.

Such variables make solid numbers on gun violence and school shootings murky. The definition of a school shooting varies by organizations, and the latest data on school-associated violent deaths in U.S. schools is from 2010, according to the Centers for Disease Control and Prevention.