Table 1 Hybrid kernel selection based on classifier-level recall (%). Table | shows the CL-recall values for the different ker- nels. For each OAA classifier, we choose the kernel with the highest CL-recall (numbers in bold in Table 1). For exam- ple, we choose the polynomial kernel for the ‘Happy or Not’ OAA classifier in our hybrid kernel approach. Note that when training individual SVM OAA classifiers, we make sure that there are a comparable number of training utterances for both classes, otherwise, the trained model will provide biased clas- sification results. For example, when training the ‘Happy or Not’ OAA classifier, ‘not happy’ utterances consist of all of the other five emotions. Therefore, we need to sample utter- ances from the five emotions with the same number as that of happy utterances. The selected kernel functions for the indi- vidual OAA classifiers are shown in Table 1.