Police departments are notoriously reluctant to make data about officer behavior available for scrutiny by outsiders. So when a Harvard economist last year asked 15 cities for as much information as they could provide for a large-scale study on the use of force, Houston Police Chief Charles McClelland says his senior staff advised him not to do it.

McClelland, however, decided to take the chance.

Houston provided detailed data on more than1 million incidents between 2010 and 2015, by far the most extensive trove of information provided to the researchers who made 20 trips from Cambridge, Mass., to Houston to examine reports, categorize them, and build a computer database for analysis.

The results, published Monday by the National Bureau of Economic Research, found that in Houston, black and Hispanic people were significantly less likely than whites to be the targets of shootings in situations where officers' lives or safety were threatened.

"Before a department can change, you have to have someone analyze your data," said McClelland, who stepped down earlier this year. "I wasn't afraid of what the answer would be. Whether good or bad, I needed to know the answers."

The answers the research provided seem to fly in the face of the narrative that's taken hold in America in the wake of high profile shootings of black men by police in Minnesota, Louisiana, and other states. The study's conclusions already have been challenged by civil rights groups.

They come as the father of a mentally ill man shot and killed by Houston police is broadly challenging HPD's internal review process in federal court. The civil rights lawsuit has been filed by Audry L. Releford, a retired Houston schoolteacher, whose son, Kenny, was killed in 2012.

He argues that HPD officials have established a custom of condoning the lethal use of force. All intentional shootings by HPD officers involving injuries or deaths - more than 150 since 2010 - have been ruled justified.

Is Houston different?

Comprehensive data on police use of lethal force is hard to find, since the nation's 18,000 independent police departments aren't required to report much to the federal government. Reviews that have been done, by news outlets such as the Washington Post and the Guardian in Britain, have found that black men make up a disproportionate share of the victims of police violence.

Last week, John Jay College's Center for Policing Equity released a large-scale study showing that black people are more likely to have all kinds of force used against them, from baton strikes to gunshots.

So is Houston just different?

Houston Police officials said that they were still reviewing the results of the study. McClelland, however, said the findings are the result of a conscious effort to educate officers about diverse cultures and train them to de-escalate conflicts. The department investigated officers if they received multiple complaints during a certain time period, McClelland said.

The Houston Police Department is also among the most diverse in the nation, with black, Hispanic, and Asian officers making up nearly half the force, local criminal justice specialists said.

The study was authored by Roland Fryer, a Harvard professor who last year was recognized by the American Economic Association as the nation's top young economist. Fryer has previous experience in Houston: He helped design and then study Apollo 20, a strategy for improving the public schools, with the cooperation of the Houston Independent School District. So, it was perhaps not surprising that he turned to Houston again.

Researchers on Fryer's team acknowledge that it would be a mistake to draw national conclusions from the one city willing to open up all its records.

Fryer's study also gathered a narrower data on officer-involved shootings data from nine other cities and counties. The study found that in situations in which police fired their weapons, blacks were no more likely to be targeted that whites. But in examining another data set, the study found that blacks were 50 percent more likely than whites to be the target of non-lethal force, such as being grabbed or slammed into a wall.

Not a fair picture

The study was greeted with skepticism by local community and civil rights groups, which said the findings didn't correspond with their experiences in the street and courts.

"In all fairness, I don't think (the study is) a fair picture of what's going on," said Johnny Mata, presiding officer of the Greater Houston Coalition for Justice, a civil rights group. "I don't care how you cut it, the short end of the stick always ends up in the communities of color."

Others critiqued the methodology of the study itself, pointing out that it's drawn from reports by police officers, which may conceal details that could put them in an unfavorable light.

Jay Jenkins, Harris County attorney for the Texas Criminal Justice Coalition, said police can be more likely to deem a situation "dangerous" - the kind of incident that the Harvard study examined - when a black person is involved. The encounters with black victims such as Alton Sterling, who was shot to death by police in Baton Rouge, La., he said, are a prime example.

The American Civil Liberties Union's Texas chapter also slammed the report for taking a narrow view.

"We've seen too many examples of bystander videos contradicting police accounts," said Jeffery Robinson, director of the ACLU Center for Justice.

"This study simply isn't helpful and does nothing to illuminate critical issues at the heart of the conversation already taking place between communities of color and police about addressing systemic bias and building out mechanisms for greater police accountability."