A method, a computer system, and a computer program product are provided for calculating a degree of similarity of users. Each user may be a preparer and an answerer of one or more quizzes. The method includes collecting one or more quizzes from one or more users as preparers and providing the collected quizzes to one or more user as answerers. The method further includes collecting one or more answering activities of one or more answerers to the provided quizzes from the one or more answerers, wherein a pair includes a preparer and an answerer. The method additionally includes calculating, for each pair, a degree of similarity of knowledge background, based on the one or more answering activities of the one or more answerers.