This study examined primary grades students' achievement on number sense tasks administered through an Internet-based formative assessment tool, Assessing Math Concepts Anywhere. Data were analyzed from 2,357 students in teachers' classrooms who had participated in a year-long professional development program on mathematics formative assessment, 1,427 students from teachers who had participated in the program in the year prior, and 9,783 students whose teachers had not participated at all. Analyses indicated that all students in the treatment group demonstrated growth, and that student achievement was influenced by the number of times the assessment was used to collect data and make instructional decisions. Further, there was a relationship between districts' socioeconomic status and growth, meaning students from impoverished backgrounds grew more than their peers.