What are they?

Systematic reviews of interventions provide comprehensive and rigorous assessments of the benefits and harms of specific interventions by searching, selecting, critically evaluating and synthesizing theevidence that is relevant to answer a specific (narrow) research or clinical question. Therefore, effectiveness systematic reviews play a pivotal role in evidence-based health care. 

We can draw on a wide range of experience of systematic reviews of interventions using rigorous gold standard Cochrane methodology including  including both randomised and non-randomised studies