No Child Left Behind (NCLB), the federal school-accountability law, is widely held to have accomplished one good thing: require states to publish test-score results in math and reading for each school in grades 3 through 8 and again in grade 10. The results appear to be telling parents whether their child’s school is doing a better job than the one across town, in the neighboring city, or across the state. But accountability works only if the yardstick used to measure performance is reasonably accurate. Unfortunately, the yardstick required by the federal law is not. Our analysis of its workings in Florida reveals it to be badly flawed and not as accurate as the measuring stick employed by the state of Florida for similar purposes.