but not in others. To be included in our analyses,studies had to have a complete comparative design, so the teaching intervention is well-defined as the difference between the intervention and the control conditions. However, when synthesizing across studies, the definition of “regular” teaching might differ across studies and, given a lack of information and a limited number of studies, there is noway to control for that. This implies that our analysis has to rely on a linear additive effects model, ignoring the fact that innovations might work out differently for different regular teaching practices