Well-Calibrated Rule Extractors

While explainability is widely considered necessary for trustworthy predictive models, most explanation modules give only a limited understanding of the reasoning behind the predictions. In pedagogical rule extraction, an opaque model is approximated with a transparent model induced using original training instances, but with the predictions from the opaque model as targets. The result is an interpretable model revealing the exact reasoning used for every possible prediction. The pedagogical approach can be applied to any opaque model and use any learning algorithm producing transparent models as the actual rule extractor. Unfortunately, even if the extracted model is induced to mimic the opaque, test set fidelity may still be poor, thus clearly limiting the value of using the extracted model for explanations and analyses. In this paper, it is suggested to alleviate this problem by extracting probabilistic predictors with well-calibrated fitness estimates. For the calibration, Venn-Abers with its unique validity guarantees, is employed. Using a setup where decision trees are extracted from MLP neural networks, the suggested approach is first demonstrated in detail on one real-world data set. After that, a large-scale empirical evaluation using 25 publicly available benchmark data sets is presented. The results show that the method indeed extracts interpretable models with well-calibrated fitness estimates, i.e., the extracted model can be used for explaining the opaque. Specifically, in the setup used, every leaf in a decision tree contains a label and a well-calibrated probability interval for the fidelity. Consequently, a user could, in addition to obtaining explanations of individual predictions, find the parts of feature space where the decision tree is a good approximation of the MLP and not. In fact, using the sizes of the probability intervals, the models also provide an indication of how certain individual fitness estimates are.