Measurement and statistical investigation are areas of mathematics visibly neglected in educational intervention research, particularly studies involving students with or at risk for mathematics difficulties (MD). This shortage is concerning given the importance these areas hold in students' pursuit of mathematical proficiency. This study investigated the initial efficacy of the "Precision Mathematics Grade 2" (PM-2) intervention, a Tier 2 (print and technology-based), integrated STEM intervention designed to increase second-grade students' mathematics achievement in the areas of measurement and statistical investigation. A total of 130 second-grade students with or at risk for MD participated in the randomized controlled trial. Students were randomly assigned within classrooms to either treatment (PM-2) or control (business-as-usual) conditions. Findings indicated a pattern of "promise" for PM-2 improving student scores on a proximal assessment of early measurement skills (Hedges' g = 0.50). Differential effects of PM-2 by initial numeracy skill were not observed for three of the outcome measures. However, moderation results were found on a curriculum-based measure, suggesting the effects of PM-2 were greatest for students with higher initial skill. Implications for supporting students' development of mathematics proficiency in areas beyond whole numbers and operations are discussed. [This paper was published in "Journal of Educational Psychology" v114 n3 p560-575 2022.]