Based on experiences from a number of large scale data- and research-informed school development projects in Denmark and Norway, led by the author, three hypotheses are discussed: that an effective way of linking research and practice is achieved (1) using a capacity building approach, that is, to collaborate in the practical school context through a problem-based learning approach (the capacity building hypothesis); (2) that data- and research-informed professional competence support and development of learning environments will increase the professional capital (individual, social and decisional capital) of teachers and school leaders (the output effect hypothesis); (3) that data- and research-informed professional competence support and development of learning environments will have a positive impact on students' learning achievements and well-being (the outcome effect hypothesis). Referring to correlational analyses of data from one of the school development projects, the outcome effect hypothesis is supported.