Output prices are mismeasured because of inadequate adjustments for changes in product quality. Thus, when quality improves, inflation will be systematically over‐stated. In this study, I find that the most commonly used indicator of the rate of inflation, the Producer Price Index, misses about 40 percent of the change in quality. However, I also find that the mismeasurement of output prices is constant over time, implying that errors of measurement are not a significant determinant of either the slowdown or recent acceleration in manufacturing productivit