1 British Journal of Mathematical and Statistical Psychology (2011), 64, 1–19 C  2010 The British Psychological Society The British Psychological Society www.wileyonlinelibrary.com Testing overall and moderator effects in random effects meta-regression Hilde M. Huizenga ∗ , Ingmar Visser and Conor V. Dolan University of Amsterdam, The Netherlands Random effects meta-regression is a technique to synthesize results of multiple studies. It allows for a test of an overall effect, as well as for tests of effects of study characteristics, that is, (discrete or continuous) moderator effects. We describe various procedures to test moderator effects: the z, t, likelihood ratio (LR), Bartlett-corrected LR (BcLR), and resampling tests. We compare the Type I error of these tests, and conclude that the common z test, and to a lesser extent the LR test, do not perform well since they may yield Type I error rates appreciably larger than the chosen alpha. The error rate of the resampling test is accurate, closely followed by the BcLR test. The error rate of the t test is less accurate but arguably tolerable. With respect to statistical power, the BcLR and t tests slightly outperform the resampling test. Therefore, our recommendation is to use either the resampling or the BcLR test. If these statistics are unavailable, then the t test should be used since it is certainly superior to the z test. 1. Introduction Consider studies that evaluate the effect of a behavioural intervention to reduce aggressive behaviour in children. Suppose that some of these studies concerned children with a high socio-economic status (SES), whereas other studies concerned children with a low SES. One may wish to establish whether the intervention is effective in general, and whether its effectiveness depends on SES. Such questions can be addressed using meta-regression analysis (Cheung, 2008; Hedges & Pigott, 2004; Normand, 1999; Thompson & Higgins, 2002; Van Houwelingen, Arends, & Stijnen, 2002). Meta-regression is a technique to synthesize results of multiple studies in order to test the overall effect, in this case overall effectiveness, and the effects of relevant study characteristics (e.g., SES). Such characteristics, which may be discrete or continuous, are called moderators. This paper focuses on the Type I error and power of various procedures to test overall effects and effects of moderators. ∗ Correspondence should be addressed to Dr Hilde M. Huizenga, Department of Psychology, University of Amsterdam, Roeterstraat 15, 1018 WB Amsterdam, The Netherlands (e-mail: h.m.huizenga@uva.nl). DOI:10.1348/000711010X522687