How should research be grouped in meta-analyses?

When conducting meta-analysis it is important to understand the detail of how research has been grouped for analysis. For example, the research on feedback needs much within-group sorting and this has led to many key interpretations. This is always based on the criteria of the researcher conducting the analysis thus making the justifications for the groupings important to include. In Visible Learning (2009) Professor Hattie provides a detailed argument for each of the groupings used, which gives validity to the subsequent effect size analyses that is conducted.