The purposes of this study were to investigate the effects of an intervention designed to improve at-risk 4th
graders’ understanding of fractions and to examine the processes by which effects occurred. The intervention
focused more on the measurement interpretation of fractions; the control condition focused more on the
part-whole interpretation of fractions and on procedures. Intervention was also designed to compensate for
at-risk students’ limitations in the domain-general abilities associated with fraction learning. At-risk students
(n 259) were randomly assigned to intervention and control. Whole-number calculation skill, domaingeneral
abilities (working memory, attentive behavior, processing speed, listening comprehension), and
fraction proficiency were pretested. Intervention occurred for 12 weeks, 3 times per week, 30 min per session,
and then fraction performance was reassessed. On each conceptual and procedural fraction outcome, effects
favored intervention over control (effect sizes 0.29 to 2.50), and the gap between at-risk and low-risk
students narrowed for the intervention group but not the control group. Improvement in the accuracy of
children’s measurement interpretation of fractions mediated intervention effects. Also, intervention effects
were moderated by domain-general abilities, but not whole-number calculation skill.