The overfit problem in inductive learning and the utility problem in speedup learning both describe a common behavior of machine learning methods: the eventual degradation of performance due to increasing amounts of learned knowledge. Plotting the performance of the changing knowledge during execution of a learning method (the performance response) reveals similar curves for several methods. The performance response generally indicates an increase to a single peak followed by a more gradual decrease in performance. The similarity in performance responses suggests a model relating performance to the amount of learned knowledge. This paper provides empirical evidence for the existence of a general model by plotting the performance responses of several learning programs. Formal models of the performance response are also discussed. These models can be used to control the amount of learning and avoid degradation of performance.