[R-meta] rule of thumb miminum number of studies per factor level meta-regression

[Lena Pollerhoff]

Dear list member, 

I am conducting meta-regressions in metafor at the moment and have a short question regarding rule of thumbs with respect to categorical predictors in meta-regression. While we are aware of one rule of thumb that meta-regressions should not be considered for fewer than ten studies per covariate (e.g., Cochrane Handbook), we were wondering whether such a rule of thumb also exists with respect to the minimum number of studies per factor level of a categorical variable?

In my case, I am conducting meta-regressions, where the number of studies per factor level are sometimes unevenly distributed: For example, k = 22, and I have one categorical predictor with three factor levels, with the first one represented by only one study, the second one by three studies, and the third one including 18 studies. 

Thanks in advance and have a nice day!
Lena Pollerhoff