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IN THE COMING DAYS, Bengals offensive coordinator Hue Jackson is likely to accept a job at the helm of an NFL team, bolstering the ranks of black head coaches. Since the league implemented the Rooney Rule in 2003, requiring owners to interview minority candidates, the number of nonwhite head coaches has grown, from two to six this past season. The policy is widely praised as a triumph of leadership; some even argue that it's no longer needed, contending that the league has solved its diversity problem.

In truth, the problem still exists -- and it might be worse than previously understood. According to new research from professors at Georgetown, George Washington, Emory and Iowa State University, white position coaches and assistants in the NFL are more than twice as likely to be promoted to coordinator than their black counterparts, regardless of their performance, experience or coaching background. One of the study's co-authors, Christopher I. Rider, says he was surprised by the magnitude of the "white coach effect" among aspiring coordinators, who aren't subject to the Rooney Rule. "Just focusing at the top is unlikely to effect much change," says Rider, an assistant professor at Georgetown.

The study's findings are especially stunning because they account for the reasons typically used to justify the race gap. Rider and his co-authors tracked the careers of more than 1,200 NFL coaches between 1985 and 2012 and tested a number of factors to see whether they could explain why white coaches -- who constituted about 72 percent of the pool -- were more likely to climb the ranks. One by one, they ticked through possibilities: the performance of each coach's team or individual unit, his age, his degree and whether he had been part of a championship staff, among other inputs. Because they had so much data, they were able to make apples-to-apples comparisons to see whether the white coach effect disappeared. It did not.

They also considered which positions the men coached when they entered the league, perhaps the most common justification for the lack of black coaches. According to their research, quarterbacks coaches are more likely to become head coaches than, say, receivers or running backs coaches. And because white players are more likely to play quarterback (a recent study found that black high school quarterbacks are 39 percent more likely to be asked to switch positions when they enter college), they are also more likely to coach the position, and then possibly become coordinators, and so forth. They accrue privilege from the moment they step foot on a field, and it only accumulates as time passes.

By controlling for this factor, Rider and his co-authors (professors James B. Wade, Anand Swaminathan and Andreas Schwab) could drill down to a simple question: If you take white and black coaches who oversee the same position, are they equally likely to advance?

The answer, according to the data, is no. The white coach is 114 percent more likely to become a coordinator. "Black coaches are less likely to be promoted than white ones, independent of their first position, their current position, their employer, their prior experience, their education and their age," the authors wrote.

And so we're left with a simple explanation: discrimination. Rider and his team estimated that it takes nine years before a white coach has a greater than 50 percent chance of becoming coordinator, compared with 14 years for a nonwhite coach. Though the salaries for various positions aren't public, the professors used back-of-the-envelope math to approximate that, over a 20-year career, a white coach is likely to earn over $20 million more than his nonwhite counterpart.

Remarkably, the white coach effect hasn't declined over the years. It's also much more pronounced at lower ends of the NFL coaching chain, according to the study. When you take into account experience and other factors, white candidates interviewing for head-coaching gigs have only a (statistically insignificant) 13 percent edge. Rider says this advantage has always been small and has not changed much since the implementation of the Rooney Rule. One possible explanation, he says, is that more people -- executives, owners, etc. -- are involved in picking head coaches, while lower level positions are typically filled with less oversight. Networking plays a bigger role -- just look at Buffalo, where head coach Rex Ryan hired his brother Rob as an assistant.

"This league is about relationships," says Herm Edwards, who was hired as head coach of the Jets in 2001 after working for the Buccaneers as an assistant to Tony Dungy. "A lot of guys who are minority coaches that come in the league, after a while they get discouraged. They realize, 'I'm going to be a position coach. And I'm OK with that.' If no one in a position of authority gives you the next move, you get stuck. There's a cycle, and it hasn't been broken."

The Fritz Pollard Alliance, a group that assists the NFL with identifying minority candidates for head-coaching jobs, also tries to identify and support potential coordinators. John Wooten, its chairman, names a few: Kirby Wilson and Sherman Smith, the running backs coaches for the Vikings and Seahawks; George Warhop, the Buccaneers' offensive line coach; Vance Joseph, the Bengals' secondary coach. Wooten says the NFL, which created a committee focused on the issue, has been a strong ally. But he admits that such efforts have their limitations. "It's very obvious to me that there are certain owners who simply want the face of their franchise to be a non-minority," he says. "We have to live with what's real."

Bias -- especially systemic bias, the sort of bias that many refuse to acknowledge until it materializes in the form of hard data -- won't be solved with a new rule or committee. But such efforts at least keep the issue in focus. If teams really want more diversity at the top, they must acknowledge its absence at the bottom, working below the surface to promote coaches of color. There were nine nonwhite coordinators on defense this season, but if Jackson ascends, there will be just two leading offenses, and countless coaches below them battling for a fair shot.