Working with a dataset from middle school students' mathematics and reading assessments, this study was conducted to gather evidence regarding effects of formative assessment on student achievement. The study used student usage statistics from an online formative assessment program to examine the effect of formative assessment on student growth scores from end-of-grade summative assessments. The major findings of this study suggest that formative assessments are positively related to student achievement in reading and mathematics. Results suggest that short-cycle reading formative assessments result in positive gains for students in reading. Both student and school-level short-cycle reading formative assessment frequency were observed to have a positive effect on student achievement in reading. The results from this study also suggest that long-cycle mathematics formative assessments may result in positive gains for students. The interaction between student and school-level long-cycle mathematics assessment frequency suggested that students who attend schools that administer a greater number of long-cycle mathematics formative assessments experience positive gains in mathematics achievement. In addition, short-cycle mathematics formative assessments seem to have a particularly stronger positive effect on the achievement of students who are economically disadvantaged.