Use of the estimated intraclass correlation for correcting differences in effect size by level

In a meta-analysis of intervention or group comparison studies, researchers often encounter the circumstance in which the standardized mean differences (d-effect sizes) are computed at multiple levels (e.g., individual vs. cluster). Cluster-level d-effect sizes may be inflated and, thus, may need to be corrected using the intraclass correlation (ICC) before being combined with individual-level d-effect sizes. The ICC value, however, is seldom reported in primary studies and, thus, may need to be computed from other sources. This article proposes a method for estimating the ICC value from the reported standard deviations within a particular meta-analysis (i.e., estimated ICC) when an appropriate default ICC value (Hedges, 2009b) is unavailable. A series of simulations provided evidence that the proposed method yields an accurate and precise estimated ICC value, which can then be used for correct estimation of a d-effect size. The effects of other pertinent factors (e.g., number of studies) were also examined, followed by discussion of related limitations and future research in this area.