Some common, fatal flaws in systematic reviews of observational studies.

When evidence from randomized controlled trials about the effectiveness and safety of an intervention is unclear, researchers may choose to review the nonrandomized evidence. All systematic reviews pose considerable challenges, and the level of methodological expertise required to undertake a useful review of nonrandomized intervention studies is both high and often severely underestimated. Using the example of the endometrial receptivity array, we review some common, critical flaws in systematic reviews of this nature, including errors in critical appraisal and meta-analysis.