A method for operating a data classifier operable to generate an element of output data in response to an element of input data, such as a neural network, is disclosed. The method includes using the data classifier to generate elements of result output data in response to elements of test input data, generating a measure of difference between each element of test output data and each corresponding element of result output data, associating the measures of difference with categories corresponding to different values of measures of difference, and, based on the number of measures of difference associated with the categories, generating a performance measure of the data classifier.