in the hour or so before turning them in to be read by an electronic scanner, erase the wrong answers and fill in correct ones. (And you always thought that no. 2 pencil was for the children to change their answers.) If this kind of teacher cheating is truly going on, how might it be detected? To catch a cheater, it helps to think like one. If you were willing to erase your students wrong answers and fill in correct ones, you prob- ably wouldnt want to change too many wrong answers. That would clearly be a tip-off. You probably wouldnt even want to change an- swers on every students test-another tip-off. Nor, in all likelihood, would you have enough time, because the answer sheets are turned in soon after the test is over. So what you might do is select a string of eight or ten consecutive questions and fill in the correct answers for, say, one-half or two-thirds of your students. You could easily memo- rize a short pattern of correct answers, and it would be a lot faster to erase and change that pattern than to go through each students an- swer sheet individually. You might even think to focus your activity toward the end of the test, where the questions tend to be harder than the earlier questions. In that way, youd be most likely to substitute correct answers for wrong ones. If economics is a science primarily concerned with incentives, it is also-fortunately-a science with statistical tools to measure how people respond to those incentives. All you need are some data. In this case, the Chicago Public School system obliged. It made available a database of the test answers for every CPS student from third grade through seventh grade from 1993 to 2000. This amounts to roughly 30,000 students per grade per year, more than 700,000 sets of test answers, and nearly 100 million individual answers. The data, organized by classroom, included each students question-by- question answer strings for reading and math tests. (The actual paper answer sheets were not included; they were habitually shredded soon