1. A method of training a classifier, the method comprising: applying a first classifier to a first set of unlabeled data to form a first set of labeled data, the first classifier capable of assigning data to classes in a first set of classes; applying a second classifier to a second set of unlabeled data to form a second set of labeled data, the second classifier capable of assigning data to classes in a second set of classes that is different from the first set of classes; and using the first set of labeled data and the second set of labeled data to retrain the first classifier to form a retrained classifier.