Defining Level 2 clusters in three-level meta-analysis

Hello community,

I've recently been exploring the fascinating method of three-level meta-analysis for handling dependent effect sizes and incorporating correlations among them. In my research, I came across an insightful guide by Cheung (2019) that highlights the benefits of this approach.

I understand that in three-level meta-analysis, it's crucial to define the groups or categories within which the effect sizes are correlated. These groups, often referred to as 'clusters,' can be at the study level or based on other factors like countries. However, I'm unsure about how to determine if a particular grouping is appropriate and well-defined.

Could you provide some guidance on evaluating the suitability of a grouping or cluster definition in three-level meta-analysis? What criteria or considerations should I keep in mind while deciding how to organize the effect sizes into these groups?

Thank you in advance for your help!

Reference:
Cheung, M.W.L. A Guide to Conducting a Meta-Analysis with Non-Independent Effect Sizes. Neuropsychol Rev 29, 387–396 (2019). https://doi.org/10.1007/s11065-019-09415-6