Effects of directionality of significance tests on the bias of accessible effect sizes.

The proportion of studies that use one-tailed statistical significance tests (π) in a population of studies targeted by a meta-analysis can affect the bias of the sample effect sizes (sample ESs, or ds) that are accessible to the meta-analyst. H. C. Kraemer, C. Gardner, J. O. Brooks, and J. A. Yesavage (1998) found that, assuming π?=?1.0, for small studies (small Ns) the overestimation bias was large for small population ESs (δ?=?0.2) and reached a maximum for the smallest population ES (viz., δ?=?0). The present article shows (with a minor modification of H. C. Kraemer et al.'s model) that when π?=?0, the small-N bias of accessible sample ESs is relatively small for δ?≤?0.2, and a minimum (in fact, nonexistent) for δ?=?0. Implications are discussed for interpretations of meta-analyses of (a) therapy efficacy and therapy effectiveness studies, (b) comparative outcome studies, and (c) studies targeting small but important population ESs. (PsycINFO Database Record (c) 2010 APA, all rights reserved)