I investigated the relationships between mathematics achievement on the Iowa Assessments and the mastery of four epistemic actions: selecting, analyzing, computing, and representing. These epistemic actions play a crucial role in characterizing mathematical reasoning and are essential for solving mathematics problems in the Iowa Assessments across different grade levels. The data analyzed included 16,644 fourth graders and 9,747 fifth graders from 2006 to 2012. The study employed the generalized DINA (deterministic inputs, noisy "and" gate) and linear regression modeling techniques. The findings of the study demonstrate a significant correlation between the mastery of the four epistemic actions and overall performance on the assessments. However, the degree of contribution from each epistemic action varied. Notably, the results indicate that fifth graders who have mastered the actions of selecting and computing are more likely to achieve high mathematics scores, regardless of their mastery levels in other epistemic actions. These findings have important pedagogical implications. It is crucial for elementary students to engage in diverse types of reasoning at early stages. However, considering the gradual development of reasoning abilities, early mastery of selecting and computing can potentially benefit students' high achievement in grades 4 and 5. By prioritizing the development of these specific skills, educators can support students in their mathematical growth and improve their overall performance on the assessments.