Why do you focus on averages and therefore ignore the complexity of the classroom and the wide variety of results?

Classrooms are places of many complexities and while there are common themes sometimes averages may not do it justice. However, the issue is that generalizability of the overall effect is an empirical issue and there are far fewer moderators* than are commonly thought (Hattie, 2009, p 10). It is important to appreciate that the 50,000 studies summarized in the book cover a very wide group of classrooms, with remarkable complexity and the aim of many of the studies was to understand these complexities. * In statistics, moderation occurs when the relationship between two variables depends on a third variable.