Of the 38 Washington, D.C., schools flagged for further investigation for cheating, 23 were found to have some degree of test violations. D.C. flagged classrooms for further investigation only if they met two of three criteria: the number of answers erased from wrong to right as compared to the state average; unusual gains in test scores; and scores within a classroom that don't fit on a bell curve. It also flagged schools that had more than one consecutive year of high erasure rates. In 2011, 70 of 5,089 tested classrooms were flagged for further investigation, including 36 public schools and 34 charter schools out of 208 tested schools in D.C.
In other cheating cases cases, statisticians say, classrooms that have had a right-to-wrong erasure ratio more than two standard deviations from the average are so irregular that they warrant a further look on their own --