The purpose of this study was to describe the Rasch model for measurement and apply the model to examine the relationship between raters, domains of written compositions, and student writing ability. Twenty raters were randomly selected from a group of 87 operational raters contracted to rate essays as part of the 1993 field test of the Georgia High School Writing Test. Each rater rated the complete set of 366 essays written by high school students and each essay was scored on 4 domains: (1) content and organization; (2) style; (3) conventions; and (4) sentence formation. The Rasch model-based FACETS computer program (J. Linacre and B. Wright, 1994) was used to examine whether there were statistically significant differences in rater severity and domain difficulty, and to explore the rater by domain interaction effect. Results indicate significant differences between raters, between domains, and a significant interaction effect between raters and domains. (Contains 4 tables, 8 figures, and 20 references.) (Author/SLD)