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Does affirmative action work? An explosive study that suggests it does not is pitting the U.S. Commission on Civil Rights against the State Bar of California in a battle over admissions data that could determine once and for all if racial preferences help or hurt minority students.

"Currently only about one in three African-Americans who goes to an American law school passes the bar on the first attempt and a majority never become lawyers at all," says UCLA law professor Richard Sander.

In an article published in the Stanford Law Review, Sander and his research team concluded several thousand would-be black lawyers either dropped out of law school or failed to pass the bar because of affirmative action.

Known as the ‘mismatch’ effect, Sander claims students who are unprepared and whose academic credentials are below the median are admitted to law schools they are unqualified to attend. If those same students instead were to go to less elite or competitive schools, more would graduate, pass the bar and become lawyers.

"This is a serious issue and we need to see more research in the area of mismatch," argues Gail Heriot, a professor of law at the University of San Diego and a member of the U.S. Commission on Civil Rights. "What we need now is more cooperation from the California Bar" Association.

Recently, a California bar committee voted 5-3 to turn down Sander’s request to use bar data collected over the last three decades on student test scores, law school admissions, academic performance and bar passage rates.

The data, considered a gold standard by affirmative action researchers, is considered key to determine if racial preferences work.

"There is no answer but to give him the information," says black civil rights attorney Leo Terrell. "What is the state bar afraid of? We need to know."

But the Bar refuses to give Sander the data.

"The release (bar exam) applicants sign does not allow us to release the information to third parties," Whitnie Henderson told FOX News. "Looking at all the information we just decided it was not something that fit within the committee’s purview."

Henderson headed the committee that rejected Sander’s request. Contrary to her statement, twice in the last 15 years the California Bar released individual information to outside researchers.

Law Professor Vikram Amar at UC Davis believes the Bar rejected Sander's request because the study is "controversial," examining the huge disparities in bar passage among different racial groups attending the same law school.

Law schools do not disclose attrition, graduation and bar passage rates to minorities admitted through preferences and have opposed pressure to do so. About 62 percent of today's top black lawyers attended the most elite U.S. law schools, according to Law Professor Richard Lempert at the University of Michigan.

Unlike Sander, Lempert believes the number of black lawyers would decrease if affirmative action ended. He says race, ethnicity and LSAT scores do not predict future income or satisfaction.

The Board of Governors of the California Bar may reconsider Sander’s request during its November meeting, but for now no one can say whether affirmative action actually does what's intended.