The Chicago Police Department doesn't pursue officers accused of excessive force as aggressively as other large departments, an expert testified Wednesday at a federal trial over the beating of a female bartender by an off-duty officer.

Dr. Steven Whitman, an epidemiologist at Chicago's Mount Sinai Hospital, told the jury that the city of Chicago upheld just 2.7 percent of the excessive-force complaints filed against officers from 1999 through 2004, well below the 6 percent national average in 2002 for departments with at least 1,000 officers.

"They are not just different by chance," said Whitman, testifying on behalf of bartender Karolina Obrycka, who has sued former Officer Anthony Abbate and the city. "There is some kind of real phenomenon that is causing it."