What are some of the common problems with the use of meta-analysis?

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).