Despite the popularity of peer assessment (PA), gaps in the literature make it difficult to describe exactly what constitutes effective PA. In a literature review, we divided PA into variables and then investigated their interrelatedness. We found that (a) PA's psychometric qualities are improved by the training and experience of peer assessors; (b) the development of domain-specific skills benefits from PA-based revision; (c) the development of PA skills benefits from training and is related to students' thinking style and academic achievement, and (d) student attitudes towards PA are positively influenced by training and experience. We conclude with recommendations for future research.

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