Meta-analysis: Principles and procedures
Open accessAbstract
Meta-analysis should be as carefully planned as any other research project, with a detailed written protocol being prepared in advance.The a priori definition of eligibility criteria for studies to be included and a comprehensive search for such studies are central to high quality meta-analysis.The graphical display of results from individual studies on a common scale is an important intermediate step, which allows a visual examination of the degree of heterogeneity between studiesDifferent statistical methods exist for combining the data, but there is no single ''correct'' method.A thorough sensitivity analysis is essential to assess the robustness of combined estimates to different assumptions and inclusion criteria.
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Only one (1%) of the 93 included articles addressed all key items.
“Only one (1%) of the 93 included articles addressed all key items [29]”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Sixteen articles (17%) provided recommendations on the key item "protocol development", with publication dates between 1994 and 2015 (median year 2009).
“Sixteen articles (17%) provided recommendations on the key item “protocol development” (Table 3), [29–44] with publication dates between 1994 and 2015 (median year of publication 2009).”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
The articles recommended that the protocol should state the objectives, hypotheses to be tested and the rationale of the review, describe eligibility criteria, define the type of studies to be included, and give the reason when including observational studies in the review.
“The articles recommended that the protocol should state the objectives, hypotheses to be tested and the rationale of the review, [29] and that it should describe eligibility criteria [29, 33, 35, 36, 38, 39, 41], define the type of studies to be included [35, 37, 42, 44], and give the reason when including observational studies in the review [35, 37, 40].”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Twenty articles (22%) gave recommendations on the key item "research question", with publication dates between 1997 and 2015 (median year 2009).
“Twenty articles (22%) gave recommendations on the key item “research question” [29, 30, 35, 36, 38–43, 45–54], with publication dates between 1997 and 2015 (median year of publication 2009).”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Nine articles (10%) made recommendations on how to perform data extraction, with publication dates between 1995 and 2016 (median year 2009).
“Nine articles (10%) made recommendations on how to perform data extraction [29, 33–35, 38, 43, 47, 48, 74], with publication dates between 1995 and 2016 (median year of publication 2009).”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
The majority of articles discussed aspects general to systematic reviews including advantages and limitations of different literature sources and databases, search tools, the importance of identifying unpublished studies, literature searching techniques including how to build a search string and reporting.
“The majority of articles discussed aspects general to systematic reviews including advantages and limitations of different literature sources and databases, search tools, the importance of identifying unpublished studies, literature searching techniques including how to build a search string and reporting [12, 29, 33–36, 46, 48, 50, 51, 53, 54, 57, 59–62, 67, 68].”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Twenty-two articles (24%) provided recommendations on the key item "study eligibility", with publication dates between 1995 and 2016 (median year 2009).
“Twenty-two articles (24%) provided recommendations on the key item “study eligibility”, [12, 29, 30, 33, 34, 37–39, 41–43, 46–48, 54, 55, 61, 65, 70–73] with publication dates between 1995 and 2016 (median year of publication 2009).”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Thirty-three articles (35%) made recommendations about the key item "search strategy", with publication dates between 1995 and 2016 (median year 2007).
“Thirty-three articles (35%) made recommendations about the key item “search strategy” [12, 29, 30, 33–36, 38, 39, 43, 46–51, 53–69], with publication dates between 1995 and 2016 (median year of publication 2007).”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Five articles gave recommendations about how to assess eligibility: it should be blind, independent, and performed by two reviewers.
“Five articles gave recommendations about how to assess eligibility: it should be blind [29, 30, 46], independent [29, 33, 46, 48], and performed by two reviewers [29, 30, 33, 48].”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Most articles considered the combination of RCTs and non-randomised designs.
“Most articles considered the combination of RCTs and non-randomised designs [29, 35, 38, 39, 49, 51, 56, 61, 75, 81].”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Several articles recommended that the data should be extracted by two independent reviewers to avoid errors, or that blinding those assessing methodological quality to the names of the authors, their institutions and the names of journals would lead to more consistent extraction.
“Several articles recommended that the data should be extracted by two independent reviewers to avoid errors [29, 33], or that blinding those assessing methodological quality to the names of the authors, their institutions and the names of journals would lead to more consistent extraction [29, 34].”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Twenty-five (27%) articles gave recommendations on item "considering different study designs", with publication dates between 1995 and 2016 (median year 2007).
“Twenty-five (27%) articles gave recommendations on item “considering different study designs” [8, 29, 34, 35, 37–39, 42, 44, 49, 51, 56, 61, 65, 70, 73, 75, 76, 77–83], with publication dates between 1995 and 2016 (median year of publication 2007).”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Recommendations were made mainly on two topics: whether different study designs should be included in a systematic review; and whether different study designs should be analysed together in a single meta-analysis.
“Recommendations were made mainly on 2 topics: whether different study designs should be included in a systematic review; [29, 34, 37–39, 49, 56, 65, 73, 75] and whether different study designs should be analysed together in a single meta-analysis [8, 34, 35, 37, 38, 42, 49, 51, 56, 61, 70, 75–78].”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
It was generally accepted that data extraction should be performed using a standardised form to avoid omissions.
“It was generally accepted that data extraction should be performed using a standardised form [29] to avoid omissions [74].”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
One article recommended neither including different study designs in systematic reviews nor combining their results in meta-analyses.
“One article recommended neither including different study designs in systematic reviews nor combining their results in meta-analyses [29].”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Twenty (22%) articles reported on item "publication bias", with publication dates between 1994 and 2012 (median year 2001).
“Twenty (22%) articles reported on item “publication bias” [16, 29, 33, 34, 36, 46, 48–50, 55, 56, 61, 70, 78, 85, 95–99], with publication dates between 1994 and 2012 (median year of publication 2001).”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Thirty-nine articles (42%) made recommendations on the assessment of quality and risk of bias of individual studies, with publication dates between 1994 and 2016 (median year 2009).
“Thirty-nine articles (42%) made recommendations on the assessment of quality and risk of bias of individual studies, [12, 16, 29, 30, 32–35, 37, 38, 43, 47–50, 52–54, 56, 64–66, 70, 71, 73, 75, 77, 82, 84–94] with publication dates between 1994 and 2016 (median year of publication 2009).”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
The majority of articles advised against the use of scales but 12 articles recommended their use.
“The majority of articles advised against the use of scales but 12 articles recommended their use [29, 30, 48–50, 52, 53, 56, 66, 77, 86, 91].”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
One common recommendation was to explore heterogeneity in sensitivity analyses, comparing studies stratified by design or clinical features.
“One common recommendation was to explore heterogeneity in sensitivity analyses, comparing studies stratified by design or clinical features [12, 16, 29, 34, 39, 51, 55, 56, 66, 77, 85, 100, 102].”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Thirty-nine articles (42%) provided recommendations on heterogeneity and its assessment, with publication dates between 1994 and 2016 (median year 2006).
“Thirty-nine articles (42%) provided recommendations on heterogeneity and its assessment, [8, 12, 16, 29, 30, 33, 34, 36, 38, 39, 41–44, 47, 49–52, 54–56, 61, 66, 70, 72, 77, 82, 85, 87, 98, 100–107] with publication dates between 1994 and 2016 (median year of publication 2006).”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Fifty-two articles (56%) provided recommendations on the statistical analysis in the context of meta-analysis of observational studies, with publication dates between 1994 and 2016 (median year 2006).
“Fifty-two articles (56%), provided recommendations on the statistical analysis in the context of meta-analysis of observational studies [8, 12, 16, 29–31, 33–36, 38, 41–44, 47, 48, 52, 54–56, 61, 66, 70, 72, 73, 77, 79–83, 85, 87, 96, 100, 101, 103–105, 107–118], with publication dates between 1994 and 2016 (median year of publication 2006).”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Only one article, published in 1997, addressed each of the 10 methodological aspects examined in the review.
“Only one article, published in 1997, addressed each of the 10 methodological aspects we examined [29].”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
The use of a random-effects model for meta-analysis of observational studies by default was frequently recommended.
“Second, the use of a random-effects model for meta-analysis of observational studies by default was frequently recommended [8, 16, 29, 34, 70, 77, 100, 101].”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Many authoritative articles providing sound advice exist, such as those by Greenland, Egger et al., and a series in Research Synthesis Methods.
“Although many authoritative articles providing sound advice exist, such as those included in this review by Greenland [85], Egger et al. [29, 97], and a series in Research Synthesis Methods [37, 40, 65, 71, 75], the inexperienced reviewer may find them difficult to identify among the many other papers with contradictory advice.”
From Methods to systematically review and meta-analyse observational studies: a systematic scoping review of recommendations (2018) · cites this paper as [29]
Visual inspection of funnel plots and Egger's tests were used to estimate publication bias.
“Visual inspection of funnel plots and Egger's tests were used to estimate publication bias [34].”
From Cognitive behavioral therapy for insomnia in patients with mental disorders and comorbid insomnia: A systematic review and meta-analysis (2022) · cites this paper as [34]
The presence of publication bias was assessed by visual inspection of the funnel plot and by using Egger's test.
“The presence of publication bias, when there were 3 or more independent estimates available, was assessed by visual inspection of the funnel plot of the meta-analyses and by using Egger's test (p b 0.05) (Egger, Smith, & Phillips, 1997).”
From Early risk and protective factors for problem gambling: a systematic review and meta-analysis of longitudinal studies (2017) · cites this paper as Egger, Smith, & Phillips, 1997
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RetractedHow to write a systematic review
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Self-citationDietary Fiber and Blood Pressure
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Natural history of untreated otitis media
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Principles of evidence based medicine
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Meta-analysis of probiotics for the prevention of traveler's diarrhea
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- Authors
- Egger, M., Smith, G. D., & Phillips, A. N.
- Affiliations
- University of Bristol (GB)
- The Royal Free Hospital (GB)
- Venue
- BMJ
- Publisher
- BMJ
- Published
- 1997-12-06
- Volume
- 315
- Issue
- 7121
- Pages
- 1533-1537
- ISSN
- 0007-1447 (ISSN-L)
- 0959-8138
- 1468-5833
- PMC
- PMC2127925
- OpenAlex
- W2155132414
- Cited by
- 2,415
- References
- 30
- Citations by year
- 2026: 57 · 2025: 96 · 2024: 137 · 2023: 128 · 2022: 96 · 2021: 93 · 2020: 110 · 2019: 90 · 2018: 104 · 2017: 92 · 2016: 95 · 2015: 116 · 2014: 116 · 2013: 136 · 2012: 125
- FWCI
- 41.86
- Type
- journal article
- Language
- en
- Open access
- green
- www.ncbi.nlm.nih.gov
- Licence
- www.bmj.com/company/legal-information/terms-conditions/legal-information/tdm-licencepolicy (tdm)
- Locations
- BMJ (publishedVersion)
- PubMed Central (publishedVersion)
- PubMed (publishedVersion)
- UCL Discovery (University College London) (submittedVersion)
- Topics
- Social Sciences › Decision Sciences › Statistics, Probability and Uncertainty › Meta-analysis and systematic reviews
- Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods in Epidemiology
- Social Sciences › Sociology and Political Science › Delphi Technique in Research
- Keywords
- meta-analysis methodology, eligibility criteria, literature searching, study heterogeneity, forest plots, sensitivity analysis
- MeSH terms
- Meta-Analysis as Topic
About these checks
Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.
Statistics and arithmetic are recomputed, not judged.
Confidence bands are shown only for fields calibrated on our annotated test set. None is calibrated yet, so no band is shown.
What counts as a claim ›How we measure confidence ›How we check this ›
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