The second strategy is to use different eligibility criteria for selecting studies that address unintended (adverse) effects compared with studies that address intended (beneficial) effects.
Different types of studies may be needed to evaluate different outcomes (Glasziou 2004). The use of different eligibility criteria specifically addresses the problem that most experimental studies (such as randomized trials) are insufficient to evaluate rare, long-term or previously unrecognized adverse effects (see Section 14.4). This approach allows a more rigorous evaluation of adverse effects, but takes more time and resources, and means that benefits and adverse effects can often not be compared directly. While randomized trials have the advantage that the allocation of interventions is made by the randomization process, non-randomized studies involve different mechanisms for allocating interventions, and these should be scrutinized during the review.