Understanding systematic reviews and meta-analysis 845 EVIDENCE BASED CHILD HEALTH 3 Understanding systematic reviews and meta-analysis AK Akobeng Arch Dis Child200590:845-848.do110.1136/adk2004.058230 This review how they selected.assessed,and analysed the s an -analyses. ed with ssess potential bias in ole o the reviere of studie s that REVIEW on a natic r nd meta the interval,and the forest plot- are introduced. y synthesising their vide of releva nical and the individua row thods used ir information av d (RCTs)are co dered to evidence of th gns a in a particular field pro DOLOGY OF A SYSTEMATIC a rative ugh still of the nigh risk of bi. and"system tic revi mit bias t s f ing a for r studies ce to THE PROBLEM WITH TRADITIONAL and rodu REVIEWS WHAT IS A META-ANALYSIS? inform may hatic AA. and ir ng 6 statistical n on a topic.'*They are also rarely explicit about Abb RCT,randd mised controlled trio EVIDENCE BASED CHILD HEALTH 3 Understanding systematic reviews and meta-analysis A K Akobeng ............................................................................................................................... Arch Dis Child 2005;90:845–848. doi: 10.1136/adc.2004.058230 This review covers the basic principles of systematic reviews and meta-analyses. The problems associated with traditional narrative reviews are discussed, as is the role of systematic reviews in limiting bias associated with the assembly, critical appraisal, and synthesis of studies addressing specific clinical questions. Important issues that need to be considered when appraising a systematic review or meta-analysis are outlined, and some of the terms used in the reporting of systematic reviews and meta￾analyses—such as odds ratio, relative risk, confidence interval, and the forest plot—are introduced. ........................................................................... ....................... Correspondence to: Dr A K Akobeng, Department of Paediatric Gastroenterology, Central Manchester and Manchester Children’s University Hospitals, Booth Hall Children’s Hospital, Charlestown Road, Blackley, Manchester, M9 7AA, UK; tony.akobeng@ cmmc.nhs.uk Accepted 22 April 2005 ....................... Health care professionals are increasingly required to base their practice on the best available evidence. In the first article of the series, I described basic strategies that could be used to search the medical literature.1 After a literature search on a specific clinical question, many articles may be retrieved. The quality of the studies may be variable, and the individual studies might have produced conflicting results. It is therefore important that health care decisions are not based solely on one or two studies without account being taken of the whole range of research information available on that topic. Health care professionals have always used review articles as a source of summarised evidence on a particular topic. Review articles in the medical literature have traditionally been in the form of ‘‘narrative reviews’’ where experts in a particular field provide what is supposed to be a ‘‘summary of evidence’’ in that field. Narrative reviews, although still very common in the medical field, have been criticised because of the high risk of bias, and ‘‘systematic reviews’’ are preferred.2 Systematic reviews apply scientific strategies in ways that limit bias to the assembly, a critical appraisal, and synthesis of relevant studies that address a specific clinical question.2 THE PROBLEM WITH TRADITIONAL REVIEWS The validity of a review article depends on its methodological quality. While traditional review articles or narrative reviews can be useful when conducted properly, there is evidence that they are usually of poor quality. Authors of narrative reviews often use informal, subjective methods to collect and interpret studies and tend to be selective in citing reports that reinforce their preconceived ideas or promote their own views on a topic.3 4 They are also rarely explicit about how they selected, assessed, and analysed the primary studies, thereby not allowing readers to assess potential bias in the review process. Narrative reviews are therefore often biased, and the recommendations made may be inap￾propriate.5 WHAT IS A SYSTEMATIC REVIEW? In contrast to a narrative review, a systematic review is a form of research that provides a summary of medical reports on a specific clinical question, using explicit methods to search, critically appraise, and synthesise the world literature systematically.6 It is particularly useful in bringing together a number of separately conducted studies, sometimes with conflicting findings, and synthesising their results. By providing in a clear explicit fashion a summary of all the studies addressing a specific clinical question,4 systematic reviews allow us to take account of the whole range of relevant findings from research on a particular topic, and not just the results of one or two studies. Other advantages of systematic reviews have been discussed by Mulrow.7 They can be used to establish whether scientific findings are consis￾tent and generalisable across populations, set￾tings, and treatment variations, or whether findings vary significantly by particular sub￾groups. Moreover, the explicit methods used in systematic reviews limit bias and, hopefully, will improve reliability and accuracy of conclusions. For these reasons, systematic reviews of rando￾mised controlled trials (RCTs) are considered to be evidence of the highest level in the hierarchy of research designs evaluating effectiveness of interventions.8 METHODOLOGY OF A SYSTEMATIC REVIEW The need for rigour in the preparation of a systematic review means that there should be a formal process for its conduct. Figure 1 sum￾marises the process for conducting a systematic review of RCTs.9 This includes a comprehensive, exhaustive search for primary studies on a focused clinical question, selection of studies using clear and reproducible eligibility criteria, critical appraisal of primary studies for quality, and synthesis of results according to a predeter￾mined and explicit method.3 9 WHAT IS A META-ANALYSIS? Following a systematic review, data from indivi￾dual studies may be pooled quantitatively and reanalysed using established statistical meth￾ods.10 This technique is called meta-analysis. The Abbreviations: RCT, randomised controlled trial Understanding systematic reviews and meta-analysis 845 www.archdischild.com