What is the total sample size of the meta analyses in Visible Learning?

Answer: Surprisingly only half the 2103 metas included information about sample size from the various studies in a meta-analysis. The total N from these 988 metas is 200,863,503 students. A projection of total N thus could be 200,863,503/ 988 * 2103 = 427,546,505. But assume 40% of the students come from overlap in the studies; this reduces the estimate to 256,527,903.  Another, and more defensible sample size estimate is to impute the average number of students per study from the 500 metas with the smallest sample data (thus, the number of studies * 92 students).  This led to an overall estimate of about 212m.  Whatever, it is a large sample size.