The excessive workload generated by the assessment of exam papers in large classes and the need to give feedback in time often constitute a rather heavy burden for teachers. The online peer assessment can contribute to reduce this workload and, possibly, to improve learning quality by assigning the assessment task to students. However, this raises the question of validity. In order to study this question, we carried out an experiment of online peer assessment in which 242 students, enrolled in 3 different courses, took part. The results show that peer assessment is equivalent to the assessment carried out by the professor in the case of exams requesting simple calculations, some mathematical reasoning, short algorithms, and short texts referring to the exact science field (computer science and electrical engineering). (Contains 3 figures and 5 tables.)