Binary classifier calibration using an ensemble of piecewise linear regression models

In this paper, we present a new nonparametric calibration method called ensemble of near-isotonic regression (ENIR). The method can be considered as an extension of BBQ (Naeini et al., in: Proceedings of twenty-ninth AAAI conference on artificial intelligence, 2015b), a recently proposed calibration method, as well as the commonly used calibration method based on isotonic regression (IsoRegC) (Zadrozny and Elkan, in: Proceedings of the ACM SIGKDD international conference on knowledge discovery and data mining 2002). ENIR is designed to address the key limitation of IsoRegC which is the monotonicity assumption of the predictions. Similar to BBQ, the method post-processes the output of a binary classifier to obtain calibrated probabilities. Thus, it can be used with many existing classification models to generate accurate probabilistic predictions. We demonstrate the performance of ENIR on synthetic and real datasets for commonly applied binary classification models. Experimental results show that the method outperforms several common binary classifier calibration methods. In particular, on the real data, we evaluated ENIR commonly performs statistically significantly better than the other methods, and never worse. It is able to improve the calibration power of classifiers, while retaining their discrimination power. The method is also computationally tractable for large-scale datasets, as it is \(O(N \log N)\) time, where N is the number of samples.