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

Michael Dewey ||@t@ @end|ng |rom dewey@myzen@co@uk
Mon Mar 28 13:37:11 CEST 2022

Dear Lena

If you have one study per level then in the model you have one parameter 
purely for that study and that study will have high influence in the 
model which you should be able to confirm from influence(yourModelHere). 
If you have few studies per level then the situation is less stark but 
the influence statistics should be your friend in clarifying what happened.


On 28/03/2022 09:40, Lena Pollerhoff wrote:
> 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
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