In short there are 3 main issues with meta-analysis as described below:
- You are comparing disparate studies so care is needed to consider possible moderators to any overall conclusion;
- Meta-analysis is focused on seeking averages and thus it is important to allow for the variance or spread of results; and,
- Studies only report past findings.
Many of the above described problems have been well considered and please refer to the Visible Learning (2009) book for an explanation of these issues (Hattie, 2009, pp. 10-12).