A critique should first and foremost make it clear which piece of writing the author is discussing. The author of the critique should mention the central idea of the piece, and whether or not it was adequately described. The critique should cite examples from the piece, and make comments about those specifically. The critique should also comment about the point of the original piece; note whether or not they agree with the writer's stance, and include their reasons for believing one way or another. A critique should be clear, concise and make it clear how they feel about the piece of writing they are discussing.
