The choice of study designs to be included in a systematic
review should be based on the type of research question being
asked and should have the goal of minimizing bias (Glasziou et
al., 2004; Oxman et al., 2006). Table 4-3 provides examples of
research questions and the types of evidence that are the most
appropriate for addressing them. RCTs can answer questions about
the efficacy of screening, preventive, and therapeutic
interventions. Although RCTs can best answer questions about the
potential harms from interventions, observational study designs,
such as cohort studies, case series, or case control studies,
may be all that are available or possible for the evaluation of
rare or long-term outcomes.7 In fact, because harms from
interventions are often rare or occur far in the future, a
systematic review of observational research may be the best
approach to identifying reliable evidence on potential rare
harms (or benefits). Observational studies are generally the
most appropriate for answering questions