"Many systematic reviews evaluating the effects of health interventions focus on evidence from RCTs, the results of which, generally, can be combined quantitatively. However, not all health care questions can be addressed by RCTs, and systematic reviews do not automatically involve statistical pooling. Meta-analysis is not always possible or sensible. For example, pooling results obtained from diverse non-randomised study types is not recommended.120 Similarly, meta-analysis of poor quality studies could be seriously misleading as errors or biases in individual studies would be compounded and the very act of synthesis may give credence to poor quality studies. However, when used appropriately, meta-analysis has the advantage of being explicit in the way that data from individual studies are combined, and is a powerful tool for combining study findings, helping avoid misinterpretation and allowing meaningful conclusions to be drawn across studies.The planned approach should be decided at the outset of the review, depending on the type of question posed and the type of studies that are likely to be available."