necessarily trump evidence from a nonrandomized study. All the evidence that is found should be clearly described and scrutinized and not just assigned to a level of a hierarchy (Glasziou et al., 2004).
The core of a systematic review is a concise and transparent synthesis of the results of the studies included in the review. The language of the review should be simple and clear so that it is usable and accessible to decision makers. The synthesis may be purely qualitative; quantitative but only descriptive, in that study results are presented in a common metric but not combined; or it may be complemented by a meta-analysis that combines the individual study results and allows statistical inference.
There are no standard guidelines for conducting or presenting the synthesis. However, the Cochrane Collaboration produces and regularly updates a methods handbook for Cochrane reviews of clinical trials that is available on the Internet (Higgins and Green, 2006). The AHRQ Effective Health Care Program is currently developing a methods manual for systematic reviews that focuses on comparative effectiveness (AHRQ, 2007a).
The synthesis should collate, describe, and summarize the following key features of the individual studies that could have a bearing on the findings:
Characteristics of the patient population, the care setting, and type of provider
Intervention (route, dose, timing, duration)
Outcome measures and timing of assessments
Quality of the evidence (i.e., risk of bias) from individual studies and possible influence on findings
Quantitative results and analyses including examination of whether the study estimates of effect are consistent across studies
Examination of potential sources of study heterogeneity, if relevant
The investigators should consider carefully if a meta-analysis is appropriate and should combine clinical judgment and a thorough understanding of the individual studies with the aggregated result. A summary estimate has the potential to mislead and lead to spurious conclusions (Editors, 2005). A detailed description of meta-analysis is beyond the scope of this report; however, an excellent review of the analytic considerations in conducting meta-analyses can be found in the text Methods for Meta-Analysis in Medical Research (Sutton et al., 2000).