Performance measures are important. They shape the future and indirectly determine the quality of human life. However, performance measures often assess something other than what researchers assume they do, and their meaning is made ambiguous by the fact that they subsume conflicting subgoals. Performance measures contain correlated errors that distort inferences, and the errors in performance measures often exceed the limitations imposed by prevalent statistical techniques. Thus, researchers should be cautious about inferring that they understand the determinants or consequences of performance.