SCRATCH any would-be education reformer among our political leaders and you’ll find someone who is easily excited by student test results. If the numbers go up then they prove the effectiveness of this or that innovation. If they go down then teachers aren’t making the effort or are accepting excuses for poor performance. And if one set of test results doesn’t provide the evidence you want, just search for another. The stakes are always high where tests like these are involved, and also at stake are the reputations of education-reformer politicians.

Nowhere is this more obvious than in New York, where optimistic and ever-improving results in the state’s own tests have been increasingly been contradicted by national test results for the same students. The New York Times reported on the latest figures last month, and the tabloid media – which has generally supported reforms driven by the current mayor, Michael Bloomberg – is starting to echo the doubts...