Created: Wed, 11 Jun 2014 10:31:00 EST
Updated: Thu, 12 Jun 2014 11:22:23 EST
(CNN) -- After Tuesday's shooting at an Oregon high school, many media outlets, including CNN, reported that there have been 74 school shootings in the past 18 months.
That's the time period since the December 2012 massacre at Sandy Hook Elementary School in Newtown, Connecticut, where 20 children and six adults were shot to death.
The statistic came from a group called Everytown for Gun Safety, an umbrella group started by former New York Mayor Michael Bloomberg, a passionate and public advocate of gun control.
Without a doubt, that number is startling.
So on Wednesday, CNN took a closer look at the list, delving into the circumstances of each incident Everytown included.
Everytown says on its web site that it gleans its information from media reports and that its list includes school shootings involving a firearm discharged inside or on school grounds, including assaults, homicides, suicides and accidental shootings.
CNN determined that 15 of the incidents Everytown included were situations similar to the violence in Oregon -- a minor or adult actively shooting inside or near a school. That works out to about one shooting every five weeks.
Some of the other incidents on Everytown's list included personal arguments, accidents and alleged gang activities and drug deals.
Here are the 15 incidents that have occurred between the Sandy Hook massacre and the latest shooting in Oregon:
1. January 10, 2013 -- Taft Union High School in Taft, California: A 16-year-old student opened fire in class. He was targeting two classmates who he felt had bullied him, law enforcement said. One student was injured. The shooter was placed into custody after a teacher and campus supervisor persuaded the student to put down his firearm.
2. January 15, 2013 -- Stevens Institute of Business and Arts in St. Louis, Missouri: A part-time student shot and injured a school employee then apparently shot himself, according to St. Louis police.
3. January 31, 2013 -- Prince Middle School in Atlanta, Georgia: A 14-year-old was shot in the head outside his school, police said. His mother told CNN affiliate WSB-TV that he was taken to the hospital. Atlanta Public Schools announced that another student had been taken into custody after the 1:50 p.m. shooting.
4. April 12, 2013 -- New River Community College in Christianburg, Virginia: An 18-year-old student drove to the school's campus inside a mall and began shooting at 1:55 p.m. on a Friday, police said. He wounded two women.
5. June 7, 2013 -- Santa Monica College in Santa Monica, California: A gunman killed four people during a rampage that began at a home near the college campus, police said. He opened fire in the residence where police discovered two bodies, then the gunman shot at a passing car and carjacked another. The gunman forced the driver to drive onto the campus and shot two people in a passing vehicle, police said. Another victim was shot outside the campus library, authorities said. The gunman was killed by authorities.
6. August 20, 2013 -- Ronald E. McNair Discovery Learning Academy, in Decatur, Georgia: A gunman fired shots and barricaded himself inside the elementary school. Antoinette Tuff, who worked in the front office, was hailed as a hero for engaging suspect Michael Brandon Hill. She managed to talk him into into giving up, and no one was injured, police said.
7. August 30, 2013 -- Carver High School in Winston-Salem, North Carolina: A student injured another during a shooting that occurred at 2:45 p.m. during a planned fire drill, CNN affiliate WXII reported.
8. October 21, 2013 -- Sparks Middle School in Sparks, Nevada: A 12-year-old opened fire with a handgun he took from his parents, police say. Jose Reyes injured two students and killed Mike Landsberry, a teacher and Afghanistan war veteran, authorities said. Reyes killed himself.
9. December 13, 2013 -- Arapahoe High School in Centennial, Colorado: Karl Halverson Pierson opened fire on a Friday inside his suburban Denver high school, police said. The 18-year-old allegedly shot Claire Davis, a 17-year-old senior, who later died at a hospital. Law enforcement said that Pierson apparently wanted to revenge against a faculty member because of a "confrontation or disagreement."
10. January 9, 2014 -- Liberty Technology Magnet High School in Jackson, Tennessee: A 16-year-old student shot
a classmate in the leg as classes were being dismissed for the day, police said, according to CNN affiliate WBBJ. The student was treated for his gunshot wound, WBBJ said.
11. January 14, 2014 -- Berrendo Middle School in Roswell, New Mexico: A 12-year-old boy walked into the school gym, pulled a shotgun out of a bag and fired at students waiting for the school day to begin, authorities said. The boy wounded two.
12. January 21, 2014 -- Purdue University in West Lafayette, Indiana: Student Cody M. Cousins, 23, opened fire in the basement of the Electrical Engineering Building, according to the university. Cousins allegedly killed Andrew F. Boldt, the school said, and both were listed as seniors.
The school's police chief said that Cousins left the building right after the shooting, and a city police officer arrested him. Cousins was "booked on a preliminary charge of murder," Purdue Police Chief John Cox said.
13. January 24, 2014 -- South Carolina State University in Orangeburg, South Carolina: A manhunt kicked off after a student Brandon Robinson, 20, was shot to death at the school, authorities said.
"He was a very nice young man," South Carolina State University President Thomas Elzey said. "And it hurts. It hurts us all."
Police arrested Justin Bernard Singleton, 19, and charged him with Robinson's killing, according to a statement from the South Carolina Law Enforcement Division.
14. May 23, 2014 -- University of California, Santa Barbara in Isla Vista, California: Elliot Rodger, 22, stabbed to death three people in his apartment, police said. Then Rodger fired into a deli and killed a young man inside the shop and shot to death two sorority sisters.
Cheng Yuan Hong, 20; George Chen, 19; Weihan Wang, 20; Katherine Cooper, 22; Veronika Weiss, 19; and Christopher Michaels-Martinez, 20, lost their lives. Others were injured.
Rodger committed suicide, leaving behind a long, hateful essay.
15. June 5, 2014 -- Seattle Pacific University in Seattle, Washington: A 26-year-old man who was not a student at the school was tackled by a student security guard after he killed one person and wounded two others, police said.
Once the suspect was on the ground, other students jumped on top of him, according to authorities.
Police told CNN affiliate KIRO that Aaron Ybarra was fascinated with school shootings and told investigators he had visited Colorado's Columbine High School where two students killed 12 students and a teacher in 1999.