Abstract: A system for improving a student's knowledge or problem solving skills that identifies errors of a student's understanding of an underlying topic or application of a requisite skill needed to answer a test question correctly. The system includes a computer with a testing software program that presents a set of two or more questions regarding a topic to a student. Associated to the topic is a set of core concepts. Each question is associated with one or more core concepts and related with the other questions so that at least one core concept matches the core concept on the other questions. When the student incorrectly answers a question, the core concepts associated with the question is reviewed and the answers to the questions containing the matching core concepts are reviewed. If a question associated with the common core concept is previously answered correctly, the core concept associated exclusively to the question answered incorrectly is deemed the source of the error.