The Cochrane Handbook for Systematic Reviews of Interventions[6] describes 3 principle reasons for undertaking a systematic review of nonrandomized trials. These reasons are as follows: (1) to examine the case for undertaking a randomized trial by providing an explicit evaluation of the weaknesses of available nonrandomized trials, (2) to provide evidence regarding an intervention that cannot be randomized, or which is extremely unlikely to be studied in randomized trials, and (3) to provide evidence of effects that cannot be adequately studied in randomized controlled trials (RCTs), such as long- term and rare outcomes.