Data-based decision making (DBDM) is an important element of educational policy in many countries, as it is assumed that student achievement will improve if teachers worked in a data-based way. However, studies that evaluate rigorously the effects of DBDM on student achievement are scarce. In this study, the effects of an intensive DBDM-intervention for Grade 4 teachers on students' mathematical achievement were investigated in a randomized controlled trial. Multilevel analyses showed that although no main effect on students' mathematical achievements was found, students who received "extended instruction" benefited significantly from the intervention. Based on the results, recommendations for the design of new DBDM-interventions and for their evaluation are presented.