Clotfelte, Ladd, & Vigdor (2008) arrived to this conclusion through the estimation of a model with student fixed effects, that minimizes possible bias, using data from North Virgina at the high school level. After ruling out alternative explanations (like prior achievements, or socio-cultural environment), they find a positive relationship between teachers’ college ratings and test scores, as well as a positive relationship between teachers’ degrees, coursework and certification and student outcomes in mathematics. Good, Grouws, & Ebmeier (1983) were among the first analytical study of the links between teachers’ education - and in particular in-service education - and student performance. This study wanted to put into evidence the impact on students of their mathematics teachers’ continuing training. To do so, teachers were assigned randomly to either a control or a treatment group.