Associated Press

CHICAGO —  Researchers have concluded in a yet-to-be published study of the economics of prostitution in Chicago that the women claim they were forced to service police officers, worked more on holidays and varied pricing based on race.

University of Chicago professor and "Freakonomics" author Steven D. Levitt and sociology professor Sudhir Venkatesh of Columbia University are the authors of the two-year study of street-level prostitution in Chicago's Roseland, Washington Park and Pullman neighborhoods.

The study has been seen by Chicago aid workers, some of whom take issue with its findings. "It's a classic example of an economist trying to tackle a very complicated problem just by looking at numbers," Samir Goswami, associate director of policy at the Chicago Homeless Coalition, told the Chicago Tribune. "It's flawed because the numbers do not explain the social situation these women are in. It's not just a business transaction."

A draft report of the study was presented last week at an economics conference in New Orleans.

Prostitutes taking part in the study — who were paid $150 a week — reported that about 3 percent of the sex acts they performed were "freebies" given to Chicago police officers to avoid arrest.

Police spokeswoman Monique Bond did not respond to requests for comment on the study's finding.

The study found full-time prostitutes made on average less than $20,000 a year. If they had a pimp, the women made a little more, even after giving up a 25 percent cut of their earnings. The women reported being beaten about once a month on average.

According to the study, Fridays were the sex trade's busiest days, Mondays the slowest. The study also found white and Hispanic men were charged more, while blacks and repeat customers paid less. Seasonal spikes in demand drove up prices, bringing more women into the market. Markets in Roseland and Pullman operated differently, the study found. In Pullman, prostitutes worked with one of four pimps, while in Roseland prostitutes worked the streets on their own.

Levitt, whose best-selling book "Freakonomics" made him a nationally known economist, and Venkatesh are refusing to comment on the study and asked its findings not be published because it was still preliminary and incomplete.

The full draft of the paper is on the University of Chicago's Web site, marked "extremely preliminary and incomplete." A university spokesman said a final version of the paper is expected to be released in April.