I would like to conduct a meta-analysis on a set of studies that have measured an outcome over a period prior to intervention and post-intervention for a control group and experimental group. A method of doing this for quantitative data was demonstrated by Morris (2008) and can be conducted in the Metafor package in R.
The only problem with Morris's proposed effect size is that it seems to be designed around the standardised mean change, which is likely not appropriate for count data. I understand that I can use the incident rate ratio (IRR) or the rate difference for count data to look at the differences between two groups. But, what if like Morris, I want to look at differences pre and post for my treatment group compared with differences pre and post for my control.
One option would be to calculate rate differences between pre and post for both groups and then to subtract one from the other. But, I am not sure whether the likely correlation between the pre and post results would make my effect size inaccurate. Another would be to potentially use the control group rate difference as a moderator, but this seems to have the same problem.
Does anyone have any suggestions on how to deal with this issue?