Over 30 percent of athletes who competed at the 2011 world championships admitted to having used banned substances in the past, according to a World Anti-Doping Agency-commissioned study.

The study, conducted by researchers from Germany's University of Tuebingen and Harvard Medical School in 2011, found that more than 30 percent of world championship participants and over 45 percent of athletes at the 2011 Pan-Arab Games said they had taken banned drugs. The researchers asked a total of 2,167 athletes whether they had used banned substances. A combined total of 5,187 athletes competed at those two events.

The 2011 world athletics championships were held in Daegu, South Korea while Qatar hosted the Pan-Arab Games that year. A process of indirect questioning was used for the study titled "Doping in Two Elite Athletics Competitions Assessed by Randomized-Response Surveys" in order to guard the athletes' anonymity. "The study shows that biological tests of blood and urine reveal only a fraction of doping cases," said Harrison Pope, Harvard Medical School professor.