Co-Training
Co-Training is another very simple but effective semi-supervised approach, proposed by Blum and Mitchell (in Blum A., Mitchell T., Combining Labeled and Unlabeled Data with Co-Training, 11th Annual Conference on Computational Learning Theory, 1998) as an alternative strategy when the dataset is a multidimensional one, and different groups of features encode different but still peculiar aspects of each class. Co-Training is effective only in scenarios where the data points can be theoretically classified using only a part of the features (even if with a light performance loss). As we're going to see, the redundancy becomes helpful in presence of an unlabeled sample, to compensate for the lack of knowledge that a single classifier might have. On the contrary, if every data point contains features that cannot be split into two separate and autonomous groups, this method is ineffective.
Co-Training theory
Let's suppose we have a labeled dataset {XL, YL} with...