How To Write A Systematic Literature Review: A Guide For .

How to write a systematic literature review:a guide for medical studentsAuthor: Rory J Piper, BMedSci(hons)Secretary, NSAMR, 2013University of Edinburgh

2How to write a systematic literature review:a guide for medical studentsWhy write a systematic review?When faced with any question, being able to conduct a robust systematic review of theliterature is an important skill for any researcher to develop; allowing identification ofthe current literature, its limitations, quality and potential. In addition to potentiallyanswering the question, the information will give guidance to the planning andsuggestion of the value of novel research.Significant experimental endeavours should be preceded by a comprehensive review ofthe subject and should exhibit the same rigour as any laboratory experiment in theassessment of both quantitative and qualitative data.Recent decades have seen the replacement of authoritative reviews by fully systematicassessment of the literature. Enthusiasts of the authoritative/opinionated review wouldargue that this method allows the reviewer to negate poorly conducted research fromboth quantitative and qualitative consideration, but such a method inappropriatelyallows the biased, imprecise and ‘unreliable’ presentation of evidence.Poorly conducted systematic reviews can mislead just like any other experimental study,yet meticulous planning and execution of the study design can minimise thecompromising factors.Important definitions:A systematic literature review attempts ‘to identify, appraise and synthesize all the empirical evidence thatmeets pre-specified eligibility criteria to answer a given research question’ (Cochrane definition, 2013).A meta-analysis is a statistical assessment of the data provided from multiple studies or sources thatattempt to ask/answer the same question.ObjectivesThis guide aims to serve as a practical introduction to: the rationale for conducting a systematic review of the literature how to search the literature qualitative and quantitative interpretation how to structure a systematic review manuscript

3Generating a hypothesisLike an experimental investigation, review of the literature and assessment of previously acquired data isconducted to test a hypothesis. In this regard, a review does not differ in attempting to test hypotheses,synthesise a new idea or reach a conclusion.A vague question is likely to lead to a vague answer. It may be advised, therefore, to limit the reviewquestion and/or aims in synchrony with a limited systematic search, discussed in the next section.Important definition:A hypothesis is a proposed explanation for an observation, one that may be investigated (scientifichypothesis) or may be carried forward (working hypothesis).Searching the literatureWhat to searchLiterature search engines work by automatic, algorithmic assessment of a search string. Despite the manyadvantages of this approach, it is often difficult to strike a balance between broad and specific searching.Subjects of high interest such as stroke are heavily investigated, so a simple search of ‘stroke’ generatesover ten-thousand results. Therefore a specific search using a well thought out collection of keywords (or‘search string’) is required to whittle down hundreds, if not thousands, of published studies in any oneparadigm.A suitable ‘search-string’ for a search engine is specific, inclusive and aware of thevariability in terminology/reporting. For the example, a search for investigating theefficacy of a neuroprotective drug: (drug-x OR drug-x-alternative-name(s) OR drug-xalternative-spelling(s)) AND (stroke OR isch(a)emia OR isch(a)emic OR cerebrovascular accident ORencephalic vascular accident). It is important to be consistent with and document yourkeywords since it is a requirement to state these in the manuscript.Further limits can be set on automatic searches, but care needs to be taken sinceinappropriately strict limits may exclude valid search results. Some authors choose tofind the literature published within a certain publication time window (justified or not),which can be helpful when reviewing ‘recent’ advances in a particular field. However,this approach is not appropriate in some systematic searches and certainly not metaanalyses, since influential studies may be eliminated from consideration.Depending on the nature or aim of the review, it may be appropriate to only considercertain types of study (such as case control, randomised controlled trial or cohort) and is

4particularly useful when there are too many studies found to be considered for a qualitative review.Where to searchLiterature search engines are powerful tools. However, there are differences between these sources andcare needs to be taken to ensure that all relevant data is obtained. Multiple engines should therefore beemployed in a systematic search;The PubMed, Medline/OvidSP (includes EMBase), Web-of-Science and BIOSIS searchengines should be considered. Furthermore, to reduce the influence of publication bias,abstracts presented at an appropriate and justified selection of conferences should bemanually searched.An obvious, but important point to remember is that there may already be a review(s)similar to that which you are considering, and effort to find these should be made (withhelp from the Cochrane library and similar resources). Discovery of such may notnecessarily negate the value of conducting a review, but may further hone its purpose.Manual scoping for studies cited by articles your review turns up, but that do not appearafter in the algorithmic search itself (‘grey literature’), provide further studies forinclusion in your review.Subscription to updates, according to the search string, from databases used allow theauthor to be alerted to the publication of new studies meeting the search criteria,allowing inclusion of studies that are published between the conducting of the searchand to the writing of the report. This is particularly important since a review is strictly‘out-of-date’ as soon as a new study emerges.Managing your findingsA meticulous search must be coupled with meticulous record keeping.Inclusion and rejection of studies found in literature researching must adhere strictly toselection criteria set out accordingly to answer the review pre-determined questions and/oraims (pre-determine to minimise selection bias).Although it is not always possible, two independent reviewers should conduct thesystematic literature search and consequent data assessment. This would further reiteratethe importance of detailed rationale and a well-communicated record of the inclusioncriteria. Rejected studies must be recorded and a third party must resolve any disagreementbetween reviewers, most appropriately by a supervisor.Commonly reviewers report that inclusion was determined on reviewing the study abstractsalone, and that certain articles were rejected if sufficient information was not presented.However, this is may be inappropriate since some journals have strict limits on abstractcontent and demand qualitative rather than quantitative abstracts (e.g. Nature). It may bemore appropriate to acquire the full-text to determine inclusion rather than reject a studybased on lack of data found in the abstract (although, of course, this is a more timeconsuming approach).The full-text versions of studies should be sought out for inclusion in the review.Institutions may not offer full-text acquisition for all journals, particularly for lessestablished/minor journals. Therefore justification for acquisition should be submitted to

5the University Library services or polite communication to the original author should be made if the fulltext is not available (or if data is missing).Language continues to be a barrier in the global communication of research and it is still found thatauthors will variably favour to cite their nation’s research. Studies should not be excluded from reviewdue to language but rather retained and translated by a professional, if possible. Abstracts are usuallypublished in English and should provide the reviewer with good indication and cause for translation.Collected data from systematic searches should be documented in an appropriate format. This isconducted in a way that suits the reviewer best. An example is provided below in which the data from asystematic search are documented in Microsoft Excel and the references retained in Mendeley referencingsoftware.Interpreting your findingsIf sufficient quantitative data is found, it may be appropriate to conduct a meta-analysis,using statistical methods to present and assess the data collected by primary studies. Thismethod is of particular use in reviewing the efficacy of a therapy or diagnostic test,provided it is measured quantitatively and is comparable between studies.Despite data handling being reliant on the sort of data collected, some forms of analysiscan be anticipated.Firstly, meta-analysis could be employed to assess the collective efficacy of a drug-xacross all the studies identified. An effectivemethod of combining different measures ofoutcomes is to use odds ratios calculated fromeach outcome with 95% confidence intervals.Analysis of statistical significance may then beemployed to estimate the magnitude of effectand heterogeneity between the studies. Thedata can then be plotted into a forest-plot,