Conceptual heterogeneity, not just design variance, makes meta-analysis impossible.

Evidence

Due to the heterogeneity of study designs, sample populations, ideologies, outcome variables, and – most importantly – the various identity concepts, a meta-analysis was not suitable, resulting in a narrative synthesis instead. — Lea Echelmeyer, Anne-Marie Slotboom, and Frank Weerman, The Putative Effect of Identity on…, 30

Categorization

• XREF: Directly bears on the reader’s own systematic review: if identity concepts are split across irreconcilable operationalizations, aggregating effect sizes may be infeasible there too. — connects to: Meta-analytic reviews must exclude regression-only studies because effect sizes require zero-order correlations — Blake M. Riek, Eric W. Mania, and Samuel L. Gaertner, Intergroup Threat and Outgroup…, 77
• QUESTION: Which specific identity-concept clusters in the 45 articles were so incommensurable that pooling was rejected?