On which studies is the “pie” from the Visible Learning based?

The pie is based on John Hattie’s analyses of the sources of variance that impact on learning. Both the Best Evidence Synthesis by Adrianne Alton-Lee (2003) and the recent meta-analyses on apportionment of variance shows that the pie is reasonably representative of the various influences. Sometimes the point is missed which is that the largest source of variance we have control over relates to teachers, not schools. And some forget to mention the phrase “that we have control over.” Of course, the pie shows the largest source of variance is among students.