Semi-supervised learning for tree-structured ensembles of RBF networks with Co-Training

Supervised learning requires a large amount of labeled data, but the data labeling process can be expensive and time consuming, as it requires the efforts of human experts. Co-Training is a semi-supervised learning method that can reduce the amount of required labeled data through exploiting the available unlabeled data to improve the classification accuracy. It is assumed that the patterns are represented by two or more redundantly sufficient feature sets (views) and these views are independent given the class. On the other hand, most of the real-world pattern recognition tasks involve a large number of categories which may make the task difficult. The tree-structured approach is an output space decomposition method where a complex multi-class problem is decomposed into a set of binary sub-problems. In this paper, we propose two learning architectures to combine the merits of the tree-structured approach and Co-Training. We show that our architectures are especially useful for classification tasks that involve a large number of classes and a small amount of labeled data where the single-view tree-structured approach does not perform well alone but when combined with Co-Training, it can exploit effectively the independent views and the unlabeled data to improve the recognition accuracy.