What type of effect size is appropriate?

There are two major types of effect sizes. One is appropriate for research that is experimental in nature such as those based on comparing groups. For example, a class that received a treatment, such as reciprocal teaching, and another class that did not receive the intervention. The other effect size approach is used to establish the impact of an intervention over item (pre-post).  They have different interpretations and it is an empirical question as to whether they differ.  In Visible Learning (2009) the majority of the effects sizes were based on research that investigated group comparisons. The “Meta^X” indicates which of these effect-sizes was used.