Economists once treated the sharp productivity slowdown of the 1970s as a puzzle to be explained by oil shocks or demographic shifts. A revisionist camp now contends that the slowdown was partly a measurement artifact. As advanced economies shifted from manufacturing toward services, national accounts—designed to tally physical output—struggled increasingly to capture value. The output of a hospital or a software firm resists the units that once made steel or wheat easy to count. If quality gains in services went unrecorded, then measured productivity understated true progress. Critics of this view concede that measurement lagged reality but deny that the gap is wide enough to erase the slowdown. They note that even sectors with straightforward output metrics decelerated. The disagreement matters because policy prescriptions diverge sharply: if the slowdown is largely illusory, forceful action to revive growth may address a problem that scarcely exists.