I am doing a meta-analysis of observational studies. As the disease I am working on has different stages, some of the studies reported ORs (95%CI) for each stage separately in comparison to a single (shared) control population. I wonder if we can combine these two ORs (and their 95%CI) given that they reflect two case groups but a single control group. Some other info:
- The sample size for each case group is different.
- We have the sample size for each of the groups but not the frequency of exposure (otherwise we could use just the raw data).
- Please note that I have no problem combining the ORs, but simply combining two ORs without taking into account that they both have the same control group will cause narrowing of the 95%CI and overweighting of the study in the analysis.