Secretary of Defense James Mattis testifies before a Senate Armed Services Committee hearing on the "Political and Security Situation in Afghanistan" on Capitol Hill in Washington, October 3, 2017.

The Pentagon is delaying release of a report by the nonpartisan RAND Corp. that rates the risk of sexual assault for troops by military base, according to Defense Department officials.

The report relied on surveys of troops and data from 2014 that have underpinned several previous reports from RAND. It was supposed to have been released months ago.

Military officials say the report is being delayed over concerns about its methodology, while advocates for troops subjected to sexual assault say the Pentagon wants to kill the report because officials do not like its conclusions.

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“Estimating risk for a large number of military installations worldwide requires some complex statistical analysis, and RAND’s 2014 military survey was not designed with this task in mind,” said Air Force Maj. Carla Gleason, a Pentagon spokeswoman. “We are working with RAND to better understand and validate its statistical methods used.”

Jeffrey Hiday, a RAND spokesman, said researchers there stand by their report and its conclusions.

"This is the first time we are hearing this particular critique," Hiday said. "We have demonstrated that the 2014 data can be used to construct installation level estimates, and the results have cleared rigorous, independent peer review.

He noted that "the entire point of the study is to estimate the risk of sexual assault at installations."

Don Christensen, an advocate for victims of sexual assault in the military, said "it's disturbing the Pentagon would hire a reputable firm like RAND and then seek to bury the results because the brass know the numbers make them look bad.”

Christensen, president of Protect Our Defenders and the former top prosecutor for the Air Force, added that “it would be nice if leadership was as concerned with finding solutions for the sexual assault crisis and holding offenders accountable as they are with manipulating data in an attempt to hide the scope of the problem.”

Sen. Kirsten Gillibrand, the New York Democrat who has been critical of the Pentagon's response to sexual assault, called for the report's immediate release.

“Sen. Gillibrand has requested that the Department of Defense release this RAND report and our office is currently waiting on a response," said Alexandria Phillips, a spokeswoman for Gillibrand. "The Department of Defense needs to be fully transparent with the findings of the report and stop protecting the status quo that harms our service members and protects predators.”

The Pentagon continues to negotiate with the research organization to before making it public, Gleason said. Hiday estimated it could be released by the end of August.

“The Department has been engaged with RAND for several months related to the methodology in the report in question,” Gleason said. “We value the innovative approaches that RAND takes to cutting-edge research and the methodology in the report has merit as a proof-of-concept. We are currently engaged with RAND to ensure the findings reflect methodology validated by the broader scientific community, and RAND is currently addressing our feedback.”

In November, the Pentagon released a report based on its own data regarding allegations of sexual assault by installation for the Air Force, Army, Marine Corps and Army. The largest bases and posts with the greatest concentrations of troops had the highest numbers of assaults, which range from groping to rape.

Naval Station Norfolk in Virginia had 270 sexual assault allegations in fiscal year 2016, the most recent reported. The Army’s Fort Hood in Texas recorded 199 reports, the Marine Corps had 169 at Camp Lejeune and the Air Force had 117 at Joint Base San Antonio Texas.

The military has struggled to address sexual assault in its ranks. For the year ending Sept. 30, 2017, the military recorded 6,769 reports of sexual assault, an increase of nearly 10% from 2016 when there were 6,172.

Military officials often attribute the increased number of allegations to greater confidence among of victims in the Pentagon’s response to the crime.