Multifaceted Rasch measurement was used in the present study with 91 student and 4 teacher raters to investigate how self- and peer-assessments work in comparison with teacher assessments in actual university writing classes. The results indicated that many self-raters assessed their own writing lower than predicted. This was particularly true for high-achieving students. Peer-raters were the most lenient raters; however, they rated high-achieving writers lower and low-achieving writers higher. This tendency was independent of their own writing abilities and therefore offered no support for the hypothesis that high-achieving writers rated severely and low-achieving writers rated leniently. On the other hand, most peer-raters were internally consistent and produced fewer bias interactions than self- and teacher-raters. Each of the four teachers was internally consistent; however, each displayed a unique bias pattern. Self-, peer-, and teacher-raters assessed "Grammar" severely and "Spelling" leniently. The analysis also revealed that teacher-raters assessed "Spelling," "Format," and "Punctuation" differently from the other criteria. It was concluded that self-assessment was somewhat idiosyncratic and therefore of limited utility as a part of formal assessment. Peer-assessors on the other hand were shown to be internally consistent and their rating patterns were not dependent on their own writing performance. They also produced relatively few bias interactions. These results suggest that in at least some contexts, peer-assessments can play a useful role in writing classes. By using multifaceted Rasch measurement, teachers can inform peer-raters of their bias patterns and help them develop better quality assessment criteria, two steps that might lead to better quality peer-assessment. (Contains 3 tables and 3 figures.)