ABSTRACT
Deep neural network (DNN), especially convolutional neural network, has achieved superior performance on image classification tasks. However, such performance is only guaranteed if the input to a trained model is similar to the training samples, i.e., the input follows the probability distribution of the training set. Out-Of-Distribution (OOD) samples do not follow the distribution of training set, and therefore the predicted class labels on OOD samples become meaningless. Classification-based methods have been proposed for OOD detection; however, in this study we show that this type of method is theoretically ineffective and practically breakable because of dimensionality reduction in the model. We also show that Glow likelihood-based OOD detection is ineffective as well. Our analysis is demonstrated on five open datasets, including a COVID-19 CT dataset. At last, we present a simple theoretical solution with guaranteed performance for OOD detection.