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The uncertainty in modeling emotions makes speech emotion recognition (SER) systems less reliable. An intuitive way to increase trust in SER is to reject predictions with low confidence. This approach assumes that an SER system is well calibrated, where highly confident predictions are often right and low confident predictions are often wrong. Hence, it is desirable to calibrate the confidence of SER classifiers. We evaluate the reliability of SER systems by exploring the relationship between confidence and accuracy, using the expected calibration error (ECE) metric. We develop a multi-label variant of the post-hoc temperature scaling (TS) method to calibrate SER systems, while preserving their accuracy. The best method combines an emotion co-occurrence weight penalty function, a class-balanced objective function, and the proposed multi-label TS calibration method. The experiments show the effectiveness of our developed multi-label calibration method in terms of ac- curacy and ECE. 

Previous studies on speech emotion recognition (SER) with categorical emotions have often formulated the task as a single-label classification problem, where the emotions are considered orthogonal to each other. However, previous studies have indicated that emotions can co-occur, especially for more ambiguous emotional sentences (e.g., a mixture of happiness and sur- prise). Some studies have regarded SER problems as a multi-label task, predicting multiple emotional classes. However, this formulation does not leverage the relation between emotions during training, since emotions are assumed to be independent. This study explores the idea that emotional classes are not necessarily independent and its implications on training SER models. In particular, we calculate the frequency of co-occurring emotions from perceptual evaluations in the train set to generate a matrix with class-dependent penalties, punishing more mistakes between distant emotional classes. We integrate the penalization matrix into three existing label-learning approaches (hard-label, multi-label, and distribution-label learn- ing) using the proposed modified loss. We train SER models using the penalty loss and commonly used cost functions for SER tasks. The evaluation of our proposed penalization matrix on the MSP-Podcast corpus shows important relative improvements in macro F1-score for hard-label learning (17.12%), multi-label learning (12.79%), and distribution-label learning (25.8%).