13.2.1.1  Including both randomized and non-randomized studies

Review authors may want to include NRS in a review because only a small number of randomized trials can be identified, or because of perceived limitations of the randomized trials. In this chapter, we strongly recommend that review authors should not make any attempt to combine evidence from randomized trials and NRS. This recommendation means that criteria for included study designs should generally specify randomized or non-randomized studies when trying to evaluate the effect of an intervention on a particular outcome. (However, a single review might consist of ‘component’ reviews that include different study designs for different outcomes, for example, randomized trials for evaluating benefits and NRS to evaluate harms; see Chapter 14.) Alternatively, where randomized trial evidence is desired but unlikely to be available, eligibility criteria could reasonably be structured to say that NRS would only be included where randomized trials are found not to be available. In time, as such a review is updated, the NRS may be dropped when randomized trials become available. Where both randomized trials and NRS of an intervention exist and, for one or more of the reasons given in Section 13.1.2, both are included in the review, these should be presented separately; alternatively, if there is an adequate number of randomized trials, comments about relevant NRS can be included in the Discussion section of a review although this is rarely particularly helpful.