systematic reviews and meta- analyses alison brettle, research fellow (information) salford centre...
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Systematic reviews and Meta-analyses
Alison Brettle, Research Fellow (Information)
Salford Centre for Nursing, Midwifery and Collaborative Research
University of Salford
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Aims
To discuss the role of the systematic reviews and meta-analyses and cover issues involved in their critical appraisal and interpretation
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Systematic Review
A review of all the literature on a particular topic, which has been systematically identified, appraised and summarised giving a summary answer.
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What is a systematic review?
An overview of primary research studies conducted according to explicit and reproducible methodology
A rigorous method of summarising research evidence
Shows what we know and don’t know about a topic area
Provides evidence of effectiveness (or not) by summarising and appraising relevant evidence
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Systematic reviews aim
To find all relevant research studies (published and unpublished)
To assess each study on basis of defined criteria
Synthesise the findings in an unbiased way
Present a balanced and impartial summary of the findings taking any flaws into consideration
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Advantages of systematic reviews
Summarise evidence, keep people up to date without reading all published research literature
Allow large amounts of data to be assimilated (eg by busy clinicians, policy makers etc)
A clearer picture by collating results of research Reduce bias – removes reviewers personal
opinions, preferences and specialist knowledge Explicit methods - allow the reader to assess how
review has been compiled More reliable conclusions because of methods
used
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Systematic review models
Medical/Health care Cochrane Collaboration, NHS Centre for
Reviews and Dissemination Usually includes “high quality” research
evidence – RCTs Often includes meta-analysis (mathematical
synthesis of results of 2+ studies that addressed same hypothesis in same way)
Social care/Social Sciences SCIE, EPPI Centre, Campbell Collaboration Often include wider range of studies including
qualitative Often narrative synthesis of evidence
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Systematic review process
Define/focus the question Develop a protocol Search the literature (possibly 2 stages scoping
and actual searches) Refine the inclusion/exclusion criteria Assess the studies (data extraction tools, 2
independent reviewers) Combine the results of the studies to produce
conclusion– can be a qualitative or quantitative (meta-analysis)
Place findings in context – quality and heterogeniety of studies, applicability of findings
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Methodology for a systematic review of randomised controlled trials1
Greenhalgh, T, BMJ 1997;315:672-675
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What type of study design?
How effective is paracetamol at reducing pain?
Does smoking increase the risk of oral cancer?
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STRONG Experimental studies/ clinical trialsRandomised controlled trialsNon-randomised controlled trials
Observational studies
CohortsCase-controlsCross-sectional surveysCase seriesCase reports
WEAK Expert opinion, consensus
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Experimental studies
Randomised controlled trial
Non-randomised controlled clinical trial
Evaluating the effectiveness of an intervention
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Observational studies
Cohort
Case-control
Cross-sectional survey
Measuring the incidence of a disease; looking at the causes of disease; determining prognosis
Looking at the causes of disease; identification of risk factors; suitable for examining rare diseases
Measuring the prevalence of a disease; examining the association
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What is a meta-analysis?
Optional part of a systematic review
Systematic reviews
Meta-analyses
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Meta-analysis
The process of using statistical methods to combine the results of different studies.
The aim is to integrate the findings, pool the data, and identify the overall trend of results
(Dictionary of Epidemiology, 1995)
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Systematic Reviews
Understanding the jargon and the blobs!
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The likelihood of something happening
V
The likelihood of something not happening
Odds Ratio, Relative RiskMeasures of risk
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Odds Ratio Graph (Blobbogram)
2 more than 1
0.5less than 1
1
Line of no significance
LEFTESS
MORIGHTE
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Odds Ratio
2 more than 1
0.5less than 1
1
Best estimate
Confidence Interval(wobble factor)
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2 more than 1
0.5less than 1
1
Odds Ratio (Blobbogram)
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Confidence Interval
Is the range within which the true size of effect (never exactly known) lies, with a given degree of assurance (95% or 99%).
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Confidence Intervals(Wobble factor)
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Confidence Interval (CI)
= the wobble factor, how sure are we about the results?
- the shorter the CI the more certain we are about the results
- if it crosses the line of 1 (no treatment effect) the intervention might not be doing any good and could be doing harm
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Heterogeneity
Clinical heterogeneity – differences in trial characteristics
Statistical heterogeneity - the variability in the reported effect sizes between studies
how similar are the results?
are the differences among the results of the trials greater than could be expected by chance alone?
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the number of people you would need to treat with a specific intervention to see one additional occurrence of a specific outcome
Number needed to treat (NNT)
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The p-value in a nutshellHow often you would see a similar result by chance, when
actually there was no effect by the drug or treatment.
p=0.001 Very unlikely 1 in 1000
p=0.05 Fairly unlikely 1 in 20
p=0.5 Fairly likely 1 in 2
p=0.75 Very likely 3 in 4
Impossible Certain Absolutely0 1
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Critical appraisal
Is the study valid? Trustworthy
What are the results? Is it useful in practice?
Relevant? Generalisable?
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Evaluating quality of systematic reviews
Is there a clearly defined question? Thorough and comprehensive search Was methodological quality assessed and
studies weighted accordingly? (Were studies reliable and valid?)
How sensitive are the results to the way the review was done – ie if you changed the inclusion criteria how would this affect results?
Interpretation of numerical results
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Further reading
Greenhalgh T (1997) How to read a paper: papers that summarize other papers (systematic reviews and meta-analyses), BMJ, 315:672-675
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Useful resources
Cochrane Collaboration http://www.cochrane.org/ http://www.cochrane.org/docs/irmg.htm
Centre for Reviews and Dissemination http://www.york.ac.uk/inst/crd/
Finding studies for systematic reviews http://www.york.ac.uk/inst/crd/revs.htm
EPPI-Centre – Stages of a review http://eppi.ioe.ac.uk/cms/Default.aspx?tabid=89
SCIE - The conduct of systematic research reviews for SCIE knowledge reviews http://www.scie.org.uk/publications/details.asp?
pubID=111