A large-scale randomized controlled trial tested the effects of researcher-developed learning games on a transfer measure of fractions knowledge. The measure contained items similar to standardized assessments. Thirty treatment and 29 control classrooms (~1500 students, 9 districts, 26 schools) participated in the study. Students in treatment classrooms played fractions games and students in the control classrooms played solving equations games. Multilevel multidimensional item response theory modeling of the outcome measure produced scaled scores that were more sensitive to the instructional treatment than standard measurement approaches. Hierarchical linear modeling of the scaled scores showed that the treatment condition performed significantly higher on the outcome measure than the control condition. The effect (d = 0.58) was medium to large (Cohen, 1992). Two appendices are included: (1) Descriptive Statistics of Pretest and Posttest Scores by Schools and Conditions; and (2) Summary of Efficacy Trial Procedures.