Patrick deHahn

USA TODAY

Most U.S. colleges -- 89% -- reported zero incidents of rape in 2015, according to American Association of University Women (AAUW) analysis of data provided by schools to the U.S. Department of Education.

Reported is the key word. Just because a school had no rape reports doesn't mean no rapes happened.

AAUW's findings very likely do not reflect the true state of sexual violence among college students, since a majority of incidents go unreported. In fact, a 2014 report by the U.S. Department of Justice estimated that 80% of student victims don't report their rape or sexual assault to police, based on data from 1995–2013.

Still, the 2015 AAUW report isn't as rosy as the previous year's. In 2014, 91% of schools reported zero rapes, based on annual crime data disclosed by more than 11,000 colleges and universities.

Related: Alarming UT-Austin report: 15% of female undergrads say they've been raped

Other studies have yielded a different picture of sexual violence on campuses. More than one in five students said they had experienced sexual abuse, along with physical abuse and threats of physical violence, a 2011 survey by Fifth & Pacific and Knowledge Networks of more than 500 college young adults aged 18-29 found.

Why the apparent disparity between reported and actual numbers? Victims have numerous reasons for not reporting their assaults. Twelve percent of victims felt their assaults were not important enough to report to police, according to the DOJ data. Other reasons cited for not reporting include personal reasons and fear of reprisal.

Related: Study: Young people still don't completely understand what constitutes sexual assault

Schools in U.S. federal financial aid programs are required to submit federal-funded yearly reports of campus violence under the Clery Act, legislation Congress passed in 1990 after a Lehigh University student was raped and killed in her dorm room in 1986. These also include updates on university safety and security policies.

The Violence Against Women Act, originally passed in 1994 and reauthorized in 2013, adds to the Clery Act requirements in having schools disclose information on rape, domestic violence, dating violence and stalking.

AAUW also found that nine percent of schools reported domestic violence incidents, and 10% reported dating violence, in 2015.

Related: Civil rights office to visit Cornell, which now has the highest number of active Title IX investigations

This story originally appeared on the USA TODAY College blog, a news source produced for college students by student journalists. The blog closed in September of 2017.