In this article, we use a two-dimensional assessment to examine the experimental impacts of a mathematics learning trajectory-oriented formative assessment program on student strategies for problems involving multiplication and division. Working from the theory that the development of students' multiplicative reasoning involves improvements in both problem-solving accuracy and sophistication of strategies used to solve problems, we designed an assessment instrument to measure both dimensions of student learning. The instrument was used to measure the impact of the Ongoing Assessment Project (OGAP), which develops teachers' capacity to regularly assess student thinking in relation to a learning progression to develop instructional responses that are based on evidence of student thinking. The results showed significant impacts of OGAP on both students' problem-solving accuracy and the sophistication of their strategy. The findings suggest that capturing both dimensions of students' multiplicative reasoning offers important information for researchers and program designers who seek to understand different dimensions of student mathematics performance.