Focusing on mathematics learning at school level, mathematics teachers have the strongest influence on student learning because they are directly working with students. Teachers are regarded as key persons of educational change. For example, a meta-analysis on student learning (Hattie, 2003) found that teachers' impact on students' learning is high: Identified factors that contribute to major sources of variation in student performance include the students (50%) and teachers (30%) as the most important factors, whereas home, schools, principals, peer effects (altogether 20%) play a less important role.