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many studies that say they can’t find a statistically significant effect of some policy change,” Hoxby says. “That doesn’t mean that there wasn’t an effect. It just means that they couldn’t find it in the data. In this study, I found estimates that are very precisely estimated around the point zero. I got a precise zero. In other words, there is no effect. ”
    This is just one study, of course. But the picture doesn’t get any clearer if you look at all the studies of class size—and there have been hundreds done over the years. Fifteen percent find statistically significant evidence that students do better in smaller classes. Roughly the same number find that students do worse in smaller classes. Twenty percent are like Hoxby’s and find no effect at all—and the balance find a little bit of evidence in either direction that isn’t strong enough to draw any real conclusions. The typical class-size study concluded with a paragraph like this:
    In four countries—Australia, Hong Kong, Scotland, and the United States—our identification strategy leads to extremely imprecise estimates that do not allow for any confident assertion about class-size effects. In two countries—Greece and Iceland—there seem to be nontrivial beneficial effects of reduced class sizes. France is the only country where there seem to be noteworthy differences between mathematics and science teaching: While there is a statistically significant and sizable class-size effect in mathematics, a class-size effect of comparable magnitude can be ruled out in science. The nine school systems for which we can rule out large-scale class-size effects in both mathematics and science are the two Belgian schools, Canada, the Czech Republic, Korea, Portugal, Romania, Slovenia, and Spain. Finally, we can rule out any noteworthy causal effect of class size on student performance in two countries, Japan and Singapore.
    Did you follow that? After sorting through thousands of pages of data on student performance from eighteen separate countries, the economists concluded that there were only two places in the world—Greece and Iceland—where there were “nontrivial beneficial effects of reduced class sizes.” Greece and Iceland? The push to lower class sizes in the United States resulted in something like a quarter million new teachers being hired between 1996 and 2004. Over that same period, per-pupil spending in the United States soared 21 percent—with nearly all of those many tens of billions of new dollars spent on hiring those extra teachers. It’s safe to say that there isn’t a single profession in the world that has increased its numbers over the past two decades by as much or as quickly or at such expense as teaching has. One country after another has spent that kind of money because we look at a school like Shepaug Valley—where every teacher has a chance to get to know every student—and we think, “There’s the place to send my child.” But the evidence suggests that the thing we are convinced is such a big advantage might not be such an advantage at all. 1

3.
    Not long ago, I sat down with one of the most powerful people in Hollywood. He began by talking about his childhood in Minneapolis. He would go up and down the streets of his neighborhood at the beginning of every winter, he said, getting commitments from people who wanted their driveways and sidewalks cleared of snow. Then he would contract out each job to other children in the neighborhood. He paid his workers the moment the job was done, with cash on hand, and collected from the families later because he learned that was the surest way to get his crew to work hard. He had eight, sometimes nine, kids on the payroll. In the fall, he would switch to raking leaves.
    “I would go and check their work so I could tell the customer that their driveway would be done the way they wanted it done,” he remembered. “There would always be one or two kids who didn’t do it well,