Confounding factors may account for any differences identified when comparing pooled results from various study designs. Due to time constraints, data was not extracted from individual papers; rather the data reported by the authors of the meta-analyses were used. Because the search strategy was designed to identify methodological papers whose primary goal was to assess the influence of study design on adverse effects, systematic reviews with embedded data comparing study designs may have been missed. The considerable heterogeneity between the comparisons of different studies suggests that any differences could be specific to particular types of interventions or certain adverse events. For example, some types of adverse events may be more easily identified by RCTs as opposed to observational studies.