I already made this question before (Is it appropriate to perform a meta-analysis of comparisons involving several communities where one of the communities was part of multiple studies?). I am making it again because after reading about network meta-analysis, as suggested in the answer that I received, I am not sure it would be helpful for my problem even if I had many studies at hand for the meta-analysis.
Here I try to elaborate my question more clearly.
I am conducting a systematic review about the effects of a policy aimed at the population level to prevent hospitalizations in older adults. The policy in question is the vaccination of children to prevent the spread of infections to older adults. In this case, the effect of the policy only affects the elderly indirectly because they are not themselves the target of the policy.
For one outcome I have 3 studies comparing communities where such policy was in place with other communities where it wasn’t.
Let’s name the communities where that policy was applied A and B, and the control communities as C, D and E.
The first study compared communities A versus C during year 2005.
The second study compared communities B versus D, during year 2005.
The third study compared communities A versus E, during years 2005, 2006 and 2007.
Please, note those are communities, not types of treatments. Policies enacted in communities A and B were similar, and communities C, D and E all represent the same absence of such policy.
I intended to perform a meta-analysis using the data from all three studies, using the estimates of all years available, but I am not sure if it would be appropriate because community A is present both in the first study and in the third study for the same year (2005).
I don’t think that the assumption of independence among studies needed for network meta-analysis holds for this case. I also understood network meta-analysis is useful to compare multiple treatments and to derive indirect comparisons between two treatments and not between two communities.
Because fixed-effect meta-analysis assume that there is a common-effect among studies and that all differences in observed effects are due to sampling error, I understand that it acknowledges to some extent the possibility of certain dependence between the effect estimates of different studies. Since fixed-effect meta-analysis is considered a special case of the random-effects meta-analysis I understand that such dependence would also be allowed in random-effects meta-analyses.