Boosting for multiclass semi-supervised learning

•We propose a boosting algorithm for multiclass semi-supervised learning, Multi-SemiAdaBoost.•It minimizes the margin cost on labeled data and the inconsistency over labeled and unlabeled data.•Multi-SemiAdaBoost uses an exponential multiclass loss function for semi-supervised learning.•Multi-SemiAdaBoost can boost any kind of base classifier.

We present an algorithm for multiclass semi-supervised learning, which is learning from a limited amount of labeled data and plenty of unlabeled data. Existing semi-supervised learning algorithms use approaches such as one-versus-all to convert the multiclass problem to several binary classification problems, which is not optimal. We propose a multiclass semi-supervised boosting algorithm that solves multiclass classification problems directly. The algorithm is based on a novel multiclass loss function consisting of the margin cost on labeled data and two regularization terms on labeled and unlabeled data. Experimental results on a number of benchmark and real-world datasets show that the proposed algorithm performs better than the state-of-the-art boosting algorithms for multiclass semi-supervised learning, such as SemiBoost (Mallapragada et al., 2009) and RegBoost (Chen and Wang, 2011).