Confusion matrix based posterior probabilities correction

An MLP classifier outputs a posterior probability for each class. With noisy data classification becomes less certain and the entropy of the posteriors distribution tends to increase, therefore providing a measure of classification confidence. However, at high noise levels entropy can give a misleading indication of classification certainty because very noisy data vectors may be classified systematically into whichever classes happen to be most noise-like. When this happens the resulting confusion matrix shows a dense column for each noise-like class. In this article we show how this pattern of misclassification in the confusion matrix can be used to derive a linear correction to the MLP posteriors estimate. We test the ability of this correction to reduce the problem of misleading confidence estimates and to increase the performance of individual MLP classifiers. Word and frame level classification results are compared with baseline results for the Numbers95 database of free format telephone numbers, in different levels of added noise.