I heard from colleagues that case studies and group studies are typically not grouped together in the same meta-analysis. Several papers discuss the topic of which type of studies are "combinable" (e.g. Egger's excellent series of methodological papers in BMJ), moreover acknowleding that the criteria according to which studies are vs aren't combinable are subjective; however, this particular aspect (combining group and case studies) is not one I found discussed.
I did find discussions of randomised controlled studies vs observational studies, which concluded that "the temptation to combine the results of studies seems to be hard to resist", and which claims RCTs should be meta-analysed separately from observational studies. But this of course only makes a distinction in the intervention type (there was or there wasn't one, resp.), while saying nothing about the sample size (group N>1, or case-study N=1) aspect of studies.
For the intervention I'm interested in, the literature consists of only few (randomised&controlled) group studies, and many single-case-studies (with N=1 or a little above 1). Is it advisable to combine all these studies together in the same meta-analysis, or are there grounds for making separate a meta-analysis for each study category?
In particular, insofar as (as Egger et al 1997 state)
a simple arithmetic average of the results from all the trials would give misleading results. (..) Methods used for meta-analysis use a weighted average of the results, in which the larger trials have more influence than the smaller ones.
, I all the more don’t understand why combining case- and group-studies is problematic, as long as the former get 'penalised', either on account on having a small N (although one can have a single group larger than each of the groups in a RCT!), or on account of "single group" leading to a decreased study-quality score - right?!