meta-analysis bias in location and selection of studies

Meta-analysis has received a mixed reception since the outset. Some people have rejected what they see as exercises in “mega-silliness,”1 while the purveyors of a highly distinguished series of meta-analyses of perinatal medical care2 have been dismissed as “an obstetrical Baader-Meinhof gang.”3 To some clinicians objecting to the findings of meta-analyses, “a tool has become a weapon.”4 At the other end of the spectrum, the application of a technique that basically consists of calculating a weighted average has been hailed as “Newtonian,”5 and it has been suggested that meta-analysis has left no place for the narrative review article.6 The truth is likely to lie somewhere between these extreme views. #### Summary points That meta-analysis holds potential problems can be illustrated by contrasting the conclusions of two meta-analyses comparing low molecular weight heparins and standard heparin in the prevention of thrombosis after surgery.7 8 One group concluded that “low molecular weight …