We examined how raters and tasks influence measurement error in writing evaluation and how many raters and tasks are needed to reach a desirable level of 0.90 and 0.80 reliabilities for children in Grades 3 and 4. A total of 211 children (102 boys) were administered three tasks in narrative and expository genres, respectively, and their written compositions were evaluated in widely used evaluation methods for developing writers: holistic scoring, productivity, and curriculum-based writing scores. Results showed that 54 and 52% of variance in narrative and expository compositions were attributable to true individual differences in writing. Students' scores varied largely by tasks (30.44 and 28.61% of variance), but not by raters. To reach the reliability of 0.90, multiple tasks and raters were needed, and for the reliability of 0.80, a single rater and multiple tasks were needed. These findings offer important implications about reliably evaluating children's writing skills, given that writing is typically evaluated by a single task and a single rater in classrooms and even in some state accountability systems.