Certain study designs are more appropriate than others for answering particular questions. Authors should consider a priori what study designs are likely to provide reliable data with which to address the objectives of their review.
Because Cochrane reviews address questions about the effects of health care, they focus primarily on randomized trials. Randomization is the only way to prevent systematic differences between baseline characteristics of participants in different intervention groups in terms of both known and unknown (or unmeasured) confounders (see Chapter 8). For clinical interventions, deciding who receives an intervention and who does not is influenced by many factors, including prognostic factors. Empirical evidence suggests that, on average, non-randomized studies produce effect estimates that indicate more extreme benefits of the effects of health care than randomized trials. However, the extent, and even the direction, of the bias is difficult to predict. These issues are discussed at length in Chapter 13, which provides guidance on when it might be appropriate to include non-randomized studies in a Cochrane review.
A practical consideration also motivates the restriction of many Cochrane reviews to randomized trials. The efforts of The Cochrane Collaboration to identify randomized trials have not been matched for the identification of other types of studies. Consequently, including studies other than randomized trials in a review may require additional efforts to identify studies and to keep the review up to date, and might increase the risk that the result of the review will be influenced by publication bias. This issue and other bias-related issues important to consider when defining types of studies (e.g. whether to restrict study eligibility on the basis of language or publication status) are discussed in detail in Chapter 10.
Specific aspects of study design and conduct should also be considered when defining eligibility criteria, even if the review is restricted to randomized trials. For example, decisions over whether cluster-randomized trials (Chapter 16, Section 16.3) and cross-over trials (Chapter 16, Section 16.4) are eligible should be made, as should thresholds for eligibility based on aspects such as use of a placebo comparison group, evaluation of outcomes blinded to allocation, or a minimum period of follow-up. There will always be a trade-off between restrictive study design criteria (which might result in the inclusion of studies with low risk of bias, but which are very small in number) and more liberal design criteria (which might result in the inclusion of more studies, but which are at a higher risk of bias). Furthermore, excessively broad criteria might result in the inclusion of misleading evidence. If, for example, interest focuses on whether a therapy improves survival in patients with a chronic condition, it might be inappropriate to look at studies of very short duration, except to make explicit the point that they cannot address the question of interest.