Table 1.

Summary of d family effect sizes, standardizers, and their recommended use.

ES	Standardizer	Use
Cohen's dpop	σ (population)	Independent groups, use in power analyses when population σ is known, σ calculated with n
Cohen's ds	Pooled SD	Independent groups, use in power analyses when population σ is unknown, σ calculated with n-1
Hedges' g	Pooled SD	Independent groups, corrects for bias in small samples, report for use in meta-analyses
Glass's Δ	SD pre measurement or control condition	Independent groups, use when experimental manipulation might affect the SD
Hedges' gav	(SD1 + SD2)/2	Correlated groups, report for use in meta-analyses (generally recommended over Hedges' grm)
Hedges' grm	SD difference scores corrected for correlation	Correlated groups, report for use in meta-analyses (more conservative then Hedges' gav)
Cohen's dz	SD difference scores	Correlated groups, use in power analyses