hardly design a better experiment in his laboratory. Just as the scientist might randomly assign one mouse to a treatment group and another to a control group, the Chicago school board effectively did the same. Imagine two students, statistically identical, each of whom wants to attend a new, better school. Thanks to how the ball bounces in the hopper, one goes to the new school and the other stays behind. Now imagine multiplying those students by the thousands. The result is a natural experiment on a grand scale. This was hardly the goal in the mind of the Chicago school officials who conceived the lottery. But when viewed in this way, the lottery offers a wonderful means of measuring just how much school choice-or, really, a better school- truly matters. So what do the data reveal? The answer will not be heartening to obsessive parents: in this case, school choice barely mattered at all. It is true that the Chicago stu- dents who entered the school-choice lottery were more likely to grad- uate than the students who didnt-which seems to suggest that school choice does make a difference. But thats an illusion. The proof is in this comparison: the students who won the lottery and went to a "better" school did no better than equivalent students who lost the lottery and were left behind. That is, a student who opted out of his     neighborhood school was more likely to graduate whether or not he actually won the opportunity to go to a new school. What appears to be an advantage gained by going to a new school isnt connected to the new school at all. What this means is that the students-and parents-who choose to opt out tend to be smarter and more aca- demically motivated to begin with. But statistically, they gained no academic benefit by changing schools. And is it true that the students left behind in neighborhood schools suffered? No: they continued to test at about the same levels as before the supposed brain drain.