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The Stata Journal
Volume 9 Number 2: pp. 197-210

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Updated tests for small–study effects in meta–analyses

Roger M. Harbord
Department of Social Medicine
University of Bristol
Bristol, UK
Ross J. Harris
Centre for Infections
Health Protection Agency
London, UK
Jonathan A. C. Sterne
Department of Social Medicine
University of Bristol
Bristol, UK
Abstract.  This article describes an updated version of the metabias command, which provides statistical tests for funnel plot asymmetry. In addition to the previously implemented tests, metabias implements two new tests that are recommended in the recently updated Cochrane Handbook for Systematic Reviews of Interventions (Higgins and Green 2008). The first new test, proposed by Harbord, Egger, and Sterne (2006, Statistics in Medicine 25: 3443–3457), is a modified version of the commonly used test proposed by Egger et al. (1997, British Medical Journal 315: 629–634). It regresses Z/√V against √V, where Z is the efficient score and V is Fisher’s information (the variance of Z under the null hypothesis). The second new test is Peters’ test, which is based on a weighted linear regression of the intervention effect estimate on the reciprocal of the sample size. Both of these tests maintain better control of the false-positive rate than the test proposed by Egger at al., while retaining similar power.
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