How to calculate weighted Hedges' g effect size in meta-analysis when some effect sizes share a control group? I am trying to do a meta-analysis using Hedges' G as the effect size. 
Questions


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*Some of the studies have different groups but the same control group. How does this change the computation and pooling of effect sizes?

*How do I calculate a weighted effect size for each study so that I can perform moderator effect analysis using SPSS?

 A: 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 7.7.3.8).

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.
References


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

