While learning new things, there is a possibility that the learning system may make mistakes. Like human beings, learning system can correct itself by identifying reasons for its failure, isolate it, explain how the particular assumption causes failure, and modifies its knowledge base. For example, while playing chess a learning system may make a wrong move and ends up with failure. Now the learning system thinks of the reasons for the failure and corrects its knowledge base. Therefore when it plays again it will not repeat the same mistake.
In his work, Active Learning with Multiple Views, Ion Muslea has used this technique to label the data. He develops a technique known as Co-EMT which is a combination of two techniques: Co-testing and Co-EM. The Co-testing method interacts with the user to label the data. If it does any mistake in labeling, it learns from the mistakes and improves. After learning, the system labels the unlabelled data extracted from a source efficiently. The labeled data constitutes what is called knowledge.