I am working on a Network Meta-analysis that is aimed at comparing different treatments (medical vs surgery vs percutaneous approach) on short as well as long-term outcomes (including cardiovascular morbidity and mortality at 1 year). We have completed a first pass literature review identifying many studies involving the follow-up of the study population referred to just one treatment (patients just referred to surgery for example) but without a comparison group (controls, or alternative active treatment). Now, considering that, as far as I know, the usual approach of multivariate meta-analysis is based on contrasts so to have direct as well as indirect comparisons, here are my questions: a) is it legitimate/appropriate (from a pure statistical stand-point of course) to include such cohort studies with no comparison group, or is it more straightfoward to exclude them ? b) If we decide to include cohort studies focused at just 1 group with no comparison group what are the pros and cons in doing that, over the alterative strategy of taking out such studies focusing on just those studies in which a comparison (RCT or not) between active treatment and controls or active treatment A vs Active treatment B have been performed ? c) In case it will turn out the inclusion of studies with just 1 group are appropriate or even encouraged, and considering that I am planning to use mvmeta STATA package for the analysis, is that something I should know on how to arrange the dataset or perform such "single arm vs contrasts" analysis ? REFs are of course very wellcome

Thank you all very much in advance,