Calibration of Medical Imaging Classification Systems with Weight Scaling

Calibrating neural networks is crucial in medical analysis applications where the decision making depends on the predicted probabilities. Modern neural networks are not well calibrated and they tend to overestimate probabilities when compared to the expected accuracy. This results in a misleading reliability that corrupts our decision policy. We define a weight scaling calibration method that computes a convex combination of the network output class distribution and the uniform distribution. The weights control the confidence of the calibrated prediction. The most suitable weight is found as a function of the given confidence. We derive an optimization method that is based on a closed form solution for the optimal weight scaling in each bin of a discretized value of the prediction confidence. We report experiments on a variety of medical image datasets and network architectures. This approach achieves state-of-the-art calibration with a guarantee that the classification accuracy is not altered.