This paper studies, using quantitative and qualitative data, the effectivity of a professional development approach in which a national summative assessment is used formatively to stimulate 19 teacher teams in Dutch secondary education to improve their data use for instruction and learning. The approach is based on conceptual similarities between 'assessment for learning (AfL)' and 'data-based decision making (DBDM)' as two approaches to formative assessment (van der Kleij et al, 2015). Cross-case analyses identify five phases of DBDM ranging from sticking to summatively assessment use (i.e., no DBDM) to formatively using assessment data to adapt classroom instruction to transferring the DBDM-cycle to other summative assessments. This study sheds innovative lights on AfL and DBDM research, practice and professional development.