I am trying to do a meta-analysis using Hedges' G as the effect size.


  1. Some of the studies have different groups but the same control group. How does this change the computation and pooling of effect sizes?
  2. How do I calculate a weighted effect size for each study so that I can perform moderator effect analysis using SPSS?

1 Answer 1


With regards to common control groups, you may want to check out 16.5.4 of the Cochrane Handbook. To quote a subset of this page:

Approaches to overcoming a unit-of-analysis error for a study that could contribute multiple, correlated, comparisons include the following.

  • Combine groups to create a single pair-wise comparison (recommended).
  • Select one pair of interventions and exclude the others.
  • Split the ‘shared’ group into two or more groups with smaller sample size, and include two or more (reasonably independent) comparisons.
  • Include two or more correlated comparisons and account for the correlation.
  • Undertake a multiple-treatments meta-analysis (see Section 16.6).

The recommended method in most situations is to combine all relevant experimental intervention groups of the study into a single group, and to combine all relevant control intervention groups into a single control group. As an example, suppose that a meta-analysis of ‘acupuncture versus no acupuncture’ would consider studies of either ‘acupuncture versus sham acupuncture’ or studies of ‘acupuncture versus no intervention’ to be eligible for inclusion. Then a study comparing ‘acupuncture versus sham acupuncture versus no intervention’ would be included in the meta-analysis by combining the participants in the ‘sham acupuncture’ group with participants in the ‘no intervention’ group. This combined control group would be compared with the ‘acupuncture’ group in the usual way. For dichotomous outcomes, both the sample sizes and the numbers of people with events can be summed across groups. For continuous outcomes, means and standard deviations can be combined using methods described in Chapter 7 (Section

With regards to pooling effect sizes for performing moderator meta analysis. There is a list of SPSS resources here. You might want to get a book like Introduction to Meta-Analysis to provide an overview of some of the many issues and calculations involved.


  • Borenstein, M., Hedges, L.V., Higgins, J.P.T. & Rothstein, H.R. (2011). Introduction to meta-analysis. John Wiley \& Sons

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