Do exams help students in study?

People said that exams can not help students with their study because exams are always asking too difficult or tricky questions. They can not test how much or how well a student has learned and whats missing in his study. In my opinion, it is true to some extent. Examsshould not be too difficult otherwise they can not do any help to all thestudents, especially those who are middle-leveled or low-leveled. Meanwhile,they cant be too easy, otherwise they can only show part of the informationabout students study. We need the well-designed exams to help us know better about our study, including the knowledge and skills we mastered and havet mastered, as well as our misunderstanding of them.

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