Reviewing the Literature Prof. Jimmy Volmink PHC Directorate Faculty of Health Science, UCT.
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Transcript of Reviewing the Literature Prof. Jimmy Volmink PHC Directorate Faculty of Health Science, UCT.
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Reviewing the Literature
Prof. Jimmy Volmink
PHC Directorate
Faculty of Health Science, UCT
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What is a literature review?
• Account of what is written on a particular subject by scholars and researchers
• Purpose: convey current knowledge and ideas as well as their strength and weaknesses
• May be: – part of essay, research report or thesis – stand alone exercise
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Annotated bibliography vs. literature review
• AB: List of sources each accompanied by a brief note (annotation) discussing one or more aspects e.g. content, usefulness, quality, etc
• LR: Discursive writing – presents themes and concepts; integrates ideas and concepts
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Objectives of literature review
1. Address a research question or thesis
2. Critically appraise research for validity
3. Synthesize and summarise current knowledge
4. Identify controversies in literature
5. Formulate questions for future research
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Information explosion
• Journals
• Textbooks
• Unpublished literature
• World Wide Web
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Reviews… reduce the load!!!
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Reviews: what do we want ?
• A readable summary of ALL the evidence
• Unbiased
• Transparent
• Up-to-date
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But is this what we get?
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Corticosteroids for prevention of Corticosteroids for prevention of respiratory distress syndromerespiratory distress syndrome
• Expert review of the literature (Robertson, BMJ 1982)
• “The evidence suggests that antenatal steroids are of value only in white males, and even for them the benefit is mainly in those of 30-32 weeks’ gestation…”
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Corticosteroids for prevention of Corticosteroids for prevention of respiratory distress syndromerespiratory distress syndrome
• Another view (Crowley, 1989)
• “The benefits of antenatal corticosteroids have been established. No further trials are necessary with the exception of certain specific situations (such as pre-eclampsia) or to establish other dosages or routes of administration.”
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Treatments for Acute MI
• Existing evidence compared with recommendations in textbooks and journals
• “advice on some life-saving therapies has been delayed for more than a decade, while other treatments have been recommended long after controlled research has shown them to be harmful.” Antman et al. JAMA 1992; 268: 240-8
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Traditional review
• Qualitative, narrative summaries of evidence on a given topic usually written by an expert in the field.
• They typically involve informal and subjective methods to collect and interpret information
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Problems with traditional reviews 1. Bias in selection of included studies
• Personal (“file drawer”) bias
• Publication bias:
– Studies with statistically significant results are:
• More likely to be published
• More likely to be published in English
• More likely to be published in journals with high citation impact factors
Easterbrooke et al, Lancet
1991
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Problems with traditional reviews 2. Indexing bias
AllRCTs
AllRCTs
OngoingOngoing
CompletedCompleted
RCTs notidentified byMEDLINE
RCTs notidentified byMEDLINE
RCTsidentified byMEDLINE
RCTsidentified byMEDLINE
UnpublishedUnpublished
PublishedPublished
Abstracts,theses, etcAbstracts,theses, etc
JournalsJournals
MEDLINEjournals(c. 4600)
1966+
MEDLINEjournals(c. 4600)
1966+
Non-MEDLINEjournals
(c. 12000)1966+
Non-MEDLINEjournals
(c. 12000)1966+
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Problems with traditional reviews 3. Variable quality of published studies
• Fourth Congress on Peer Review in Biomedical Publication :
“Medical journals are full of serious methodological errors “
“Journal editors are giving no time, energy and thought to their craft”
“Studies are published that reach false conclusions”BMJ, Sept. 2001:323
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Problems with traditional reviews 4. Effect of Study Design
PURPOSE: To assess the evidence of the effectiveness of vitamin supplementation, specifically vitamins A, C, and E; beta-carotene; folic acid; antioxidant combinations; and multivitamin supplements, in preventing cardiovascular disease.
STUDY SELECTION: The researchers selected English-language reports of randomized trials and cohort studies that assessed vitamin supplementation in western populations and reported incidence of or death from cardiovascular events.
CONCLUSIONS: Some good-quality cohort studies have reported an association between the use of vitamin supplements and lower risk for cardiovascular disease. Randomized, controlled trials of specific supplements, however, have failed to demonstrate a consistent or significant effect of any single vitamin or combination of vitamins on incidence of or death from cardiovascular disease.
Ann Intern Med. 2003;139:56-70.
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Problems with traditional reviews 5. Effect of Study Size
“ … we still have no clear evidence that beta-blockers improve long-term survival after infarction despite almost 20 years of
clinical trials.”
JRA Mitchell. BMJ 1981;282:1565-70
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The Science of Research Synthesis
“If science consisted of nothing but the laborious accumulation of facts, it would soon come to a standstill...…Two processes are at work side by side, the reception of new material and the digestion and assimilation of the old; and both are essential…”
Lord Rayleigh 1884
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A Systematic ReviewHow is it different?
“A review in which bias has been reduced by the:
– systematic identification,
– appraisal,
– synthesis
– statistical aggregation (if relevant)
of all relevant studies on a specific topic according to a
predetermined and explicit method.”Moher et al.
Lancet; 354: 1896-900
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Steps in conducting a systematic review
1. Formulate topic/question
2. Decide upon selection criteria for studies
3. Determine search strategy
4. Critically appraise research
5. Extract data
6. Synthesise and summarise data
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1. The review question
• Do betablockers compared to placebo decrease morbidity and mortality in people with hypertension?
• Does the administration of Bacillus Calmette-Guerin (BCG) influence the risk of tuberculosis and all-cause mortality in populations exposed to infection?
• PICO Method
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2. Selection criteria for studies
• Types of participants– People receiving BCG vaccination,
irrespective of age
• Types of interventions– BCG vaccination given by any route.
• Types of outcome measures– Death– Tuberculosis
• Types of studies – RCT
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3. Search strategy
• Scope of search– Published vs. unpublished vs. grey literature– Peer-reviewed vs. non peer-reviewed– Electronic vs. paper based sources
• Search should be as comprehensive as possible
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Ingredients of a comprehensive literature search
• Electronic databases (highly sensitive strategy)– MEDLINE/PUBMED– EMBASE– Cochrane Databases – CENTRAL – Other e.g. CINAHL, AIDSLINE, LILACS, etc.
• Handsearching of journals• Reference lists• Conference proceedings• Personal communication
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4. Critical appraisal of research
• Different types of clinical research• Need to understand strengths and
limitations of various study designs• Know what aspects of a study to assess to
determine likelihood of bias
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5. Extract data
• Pre-specify items of information to be collected
• Data extraction form
• Independent data extraction
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6. Synthesise and summarise data
• Qualitative (narrative)
• Quantitative synthesis (uses statistics)
• Some statistical knowledge will be needed
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Meta-analysisPooling the results
• A statistical procedure that pools the results of several independent studies considered sufficiently similar (homogeneous)
• Not appropriate if studies are too dissimilar (heterogeneous)
• Provides a quantitative summary of the overall treatment effect
• Visual display called a “forest plot”
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• Estimate of effect
• Confidence intervals
• Weighting
• Overall estimate of effect
Meta-analysis and the Forest Plot
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Meta-analysis Beta-carotene and cardiovascular mortality
0.1 0.5 0.75 1 1.25 1.5 1.75
Male health workers
Social insurance, men
Male chemical workers
Hyperlipidaemic men
Nursing home residents
Social insurance, women
Male physicians
Male smokers
(Ex)-smokers, asbestos workers
Trials
Cohort studies
Skin cancer patients
USA
Finland
Switzerland
USA
USA
Finland
Cohorts combined
Trials combined
Finland
USA
USA
USA
Relative risk (95% CI)
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Acknowledging your sources
• Why bother:– Honesty and transparency– Show research done to reach conclusions– Allow readers to identify and retrieve
references
• Plagiarism – “taking, using and passing off as your own work the ideas or words of another”– See UCT policy
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Types of Citation
Two main types:
1. Harvard: author-date style– …as one author has put it “the darkest days were
still ahead” (Weston 1988, p.45)
2. Vancouver: footnote/endnote style set by the International Committee of Medical Editors– …as one author has put it “the darkest days were
still ahead”[1]• [ ] , ( ) or supercript …ahead”1 all acceptable
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In summary…
Traditional vs. systematic review
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Traditional review
• Qualitative, narrative summaries of evidence on a given topic usually written by an expert in the field. Typically, involve informal and subjective methods to collect and interpret information
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Systematic review
“A review in which bias has been reduced by the systematic identification, appraisal, synthesis, and, if relevant statistical aggregation of all relevant studies on a specific topic according to a predetermined and explicit method.”
Moher et al. Lancet 1999; 354: 1896-900
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Cochrane review
• A systematic review produced by the Cochrane Collaboration (www.cochranelibrary.com)– Standardised format– Extensive peer review– Published electronically on the Cochrane Library (and indexed in
Medline)– Invites comments and criticism– Kept up-to-date
• Quality and reporting on average better than other systematic reviews ….but not without flaws!
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Summary: Types of reviews
All reviews
Systematic reviews
Meta-analyses