Meta-analysis: Difference between revisions

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When analyzing a meta-regression of dichotomous independent variables, the "results of meta-regression  analyses are most usefully expressed as ratios of odds ratios (or risk  ratios)."<ref name="CochraneHandbook>The Cochrane Collaboration. [http://www.cochrane-handbook.org/ Cochrane Handbook]</ref>
When analyzing a meta-regression of dichotomous independent variables, the "results of meta-regression  analyses are most usefully expressed as ratios of odds ratios (or risk  ratios)."<ref name="CochraneHandbook>The Cochrane Collaboration. [http://www.cochrane-handbook.org/ Cochrane Handbook]</ref>


Meta-regression can be performed with the [http://cran.r-project.org/web/packages/rmeta/ rmeta package] of the [[R (programming language)|R programming language]].<ref>Lumley,T. (2009) [http://cran.r-project.org/web/packages/rmeta/ rmeta: Meta-analysis]</ref>
Meta-regression can be performed with the [http://cran.r-project.org/web/packages/rmeta/ rmeta package]<ref>Lumley,T. (2009) [http://cran.r-project.org/web/packages/rmeta/ rmeta: Meta-analysis]</ref> of the [[R (programming language)|R programming language]] as described by Everitt and Hothorn<ref>Everitt B, Hothorn, T. (2009) [http://cran.r-project.org/web/packages/HSAUR2/ HSAUR2]</ref><ref name="isbn1-4200-7933-6">{{Cite book  | last1 = Everitt | first1 = Brian | last2 = Hothorn | first2 = Torsten | title = A Handbook of Statistical Analyses Using R, Second Edition | date =  | publisher = Chapman  Hall/CRC | location =  | isbn = 1-4200-7933-6 | pages =  }}</ref>.


===Network meta-analysis===
===Network meta-analysis===

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Meta-analysis is defined as "a quantitative method of combining the results of independent studies (usually drawn from the published literature) and synthesizing summaries and conclusions which may be used to evaluate therapeutic effectiveness, plan new studies, etc., with application chiefly in the areas of research and medicine."[1]

A meta-analyses is a subset of systematic reviews in which the results of the studies are numerically pooled.

Standards for the reporting of meta-analyses exist.[2]

Validity of meta-analysis

Studies on the validity of meta-analyses conflict.[3][4][5] Some of the conflict may be due to the methods used to compare the meta-analyses.[6]

Methods of meta-analysis

Guidelines are available for the conduct[7] and reporting[2] of meta-analyses.

Searching for studies

Meta-analyses vary in the extent of their searches for underlying studies. [8] There is debate on how extensive should be the search for studies as there is are diminishing returns with extensive searching. Some studies suggest limiting searches[9][10][11] while other studies advocate exhaustive searches[12][13][14][15][16][17] including unpublished studies[18][19].

There is not a consensus on what details of searching should be reported in a meta-analysis.[20]

Selecting studies for inclusion

Conflict in selection of trials to be included in the meta-analysis can affect the conclusions of a meta-analysis.[21][22][23]

Although meta-analyses in general are very inclusive, arguments exist for only including the best trials.[24]

Assessing the quality of trials

For more information, see: Randomized controlled trial.


Cochrane bias scale

The Cochrane Collaboration uses a six item tool.[25]

Jadad score

The Jadad score may be used to assess quality and contains three items:[26]

  1. Was the study described as randomized (this includes the use of words such as randomly, random, and randomization)?
  2. Was the study described as double blind?
  3. Was there a description of withdrawals and dropouts?

Each question is scored one point for a yes answer. In addition, for questions and 2, a point is added if the method was appropriate and a point is deducted if the method is not appropriate (e.g. not effectively randomized or not effectively double-blinded).

Statistical methods

Studies are usually statistically combined by a method such as the DerSimonian and Laird.[27]

Statistical packages are available from the Cochrane Collaboration (http://www.cc-ims.net/revman) and for R (programming language) (rmeta and HSAUR2).

Studies with groups having zero events

Excluding studies with zero events total events (zero-total-event trials) or zero events in one treatment group (zero-event trials) may exaggerate effect sizes.[28][29] An alternative is to use a continuity correction.[30] Rather than using a constant continuity correction, less bias may occur by correcting with either[31]

  • "empirical estimate of the pooled effect size from the remaining studies in the meta-analysis."
  • "a function of the reciprocal of the opposite group arm size"

For an example of continuity correction using the second method above:[28]

  • S is the sum of corrections for event and no event cells (usually S=1 in a zero-event trial and S=2 in a zero-total-event trial)
  • R is the ratio of group sizes (R=1 if both groups are the same)
  • For a zero-event trial with equal group sizes
    • The correction in the larger experimental group is R/S*(R + 1). This becomes 1/1*(1 + 1) = 1
    • The correction in the smaller experimental group is 1/S*(R + 1). This becomes 1/1*(1 + 1) = 1
  • For a zero-event-total trial with equal group sizes
    • The correction in the larger experimental group is R/S*(R + 1). This becomes 1/2*(1 + 1) = 0.5
    • The correction in the smaller experimental group is 1/S*(R + 1). This becomes 1/2*(1 + 1) = 0.5

Displaying results

Study results may be grouped and displayed with a Forest plot.

(CC) Photo: Robert Badgett
Forest Plot showing meta-analysis of randomized controlled trials of differing target glucose control and mortality for diabetes mellitus type 2. Note the heterogeneity (P<0.05 and high I2 in circled in red) due to increased death when the glycosylated hemoglobin A (Hb A1c) target was 6.0% in the ACCORD trial[32]

Measuring consistency of study results

Consistency can be statistically tested using either the Cochran's Q or I2.[33] The I2 is the "percentage of total variation across studies that is due to heterogeneity rather than chance."[33] These numbers are usually displayed for each group of studies on a Forest plot.

In interpreting of the Cochran's Q, heterogeneity exists if its p-value is < 0.05 or possibly if < 0.10[34][35].

The following has been proposed for interpreting I2:[33]

  • Low heterogeneity is I2 = 25%
  • Moderate heterogeneity is I2 = 50%
  • High heterogeneity is I2 = 75%

or according to the Handbook of the Cochrane Collaboration:[36]

  • 0%-40%: might not be important
  • 30%-60%: may represent moderate heterogeneity
  • 50%-90%: may represent substantial heterogeneity
  • 75%-100%: considerable heterogeneity

Statistical methods exist for assessing the importance of subgroups.[37]

Variations on meta-analysis

Cumulative meta-analysis

Cumulative meta-analysis has been used to show that 25 off 33 randomized controlled trials of streptokinase not necessary[38] and have shown the delay in adoption of evidence by experts[39].

Individual patient data meta-analysis

An individual patient data meta-analysis is "where analyses are done using original data and outcomes for each person enrolled in relevant studies; these results are then pooled in one analysis as if patients were in a single large study."[40]

Individual patient data meta-analysis (IPD meta-analysis) may have more long lasting results than other meta-analyses.[41]

Meta-regression

Meta-regression allows simultaneous comparison of multiple sources of heterogeneity.[42][43][44][45]

Examples of meta-regression analysis are:
1. McAlister FA, Wiebe N, Ezekowitz JA, Leung AA, Armstrong PW (2009). "Meta-analysis: beta-blocker dose, heart rate reduction, and death in patients with heart failure.". Ann Intern Med 150 (11): 784-94. PMID 19487713.

2. Briel M, Ferreira-Gonzalez I, You JJ, Karanicolas PJ, Akl EA, Wu P et al. (2009). "Association between change in high density lipoprotein cholesterol and cardiovascular disease morbidity and mortality: systematic review and meta-regression analysis.". BMJ 338: b92. DOI:10.1136/bmj.b92. PMID 19221140. PMC PMC2645847. Research Blogging.

3. Emerging Risk Factors Collaboration. Erqou S, Kaptoge S, Perry PL, Di Angelantonio E, Thompson A et al. (2009). "Lipoprotein(a) concentration and the risk of coronary heart disease, stroke, and nonvascular mortality.". JAMA 302 (4): 412-23. DOI:10.1001/jama.2009.1063. PMID 19622820. Research Blogging.


When analyzing a meta-regression of dichotomous independent variables, the "results of meta-regression analyses are most usefully expressed as ratios of odds ratios (or risk ratios)."[7]

Meta-regression can be performed with the rmeta package[46] of the R programming language as described by Everitt and Hothorn[47][48].

Network meta-analysis

A network meta-analysis[49] and Bayesian hierarchical models[50] pool studies in order to compare to treatments that have not been directly compared.[51] Network meta-analyses are commonly not well performed[52]and can have misleading conclusions.[53][54][55]

Network meta-analyses can be conducted with Bugs and OpenBugs software.

Factors associated with higher quality meta-analyses

Meta-analyses by the Cochrane Collaboration tend to be of higher quality.[56]

Individual data meta-analyses, in which the records from individual patients are pooled together into one dataset, tend to have more stable conclusions.[41]

Factors associated with lower quality meta-analyses

About a third of meta-analyses that happen to precede large randomized controlled trials will conflict with the results of the trial.[3]

Conflict of interest

Meta-analyses produced with a conflict of interest are more likely to interpret results as positive.[57]

Publication bias

Publication bias against negative studies may threaten the validity of meta-analyses that are positive and all the studies included within the meta-analysis are small.[58][59]

In performing a meta-analysis, a file drawer[60]or a funnel plot analysis[59][61] may help detect underlying publication bias among the studies in the meta-analysis.

Outcome reporting bias

Meta-analyses in which a smaller proportion of included trials provide raw data for inclusion in the meta-analysis are more likely to be positive.[62] This may be due a bias against reporting negative results.[63]

Problems with meta-analyses

Obsolescence

The conclusions of meta-analyses may be mitigated by research published after the search date of the meta-analysis. This may occur by the time the meta-analysis has been published.[64][65] Strategies have been developed for updating meta-analyses.[66]

References

  1. ↑ National Library of Medicine. Meta-analysis. Retrieved on 2007-12-06.
  2. ↑ 2.0 2.1 Moher D, Liberati A, Tetzlaff J, Altman DG (July 2009). "Preferred Reporting Items for Systematic Reviews and Meta-Analyses: The PRISMA Statement". Ann. Intern. Med.. PMID 19622511. [e] Cite error: Invalid <ref> tag; name "pmid19622511" defined multiple times with different content
  3. ↑ 3.0 3.1 LeLorier J, Grégoire G, Benhaddad A, Lapierre J, Derderian F (August 1997). "Discrepancies between meta-analyses and subsequent large randomized, controlled trials". N. Engl. J. Med. 337 (8): 536–42. PMID 9262498. [e] Cite error: Invalid <ref> tag; name "pmid9262498" defined multiple times with different content
  4. ↑ Villar J, Carroli G, Belizán JM (March 1995). "Predictive ability of meta-analyses of randomised controlled trials". Lancet 345 (8952): 772–6. PMID 7891492. [e]
  5. ↑ Cappelleri JC, Ioannidis JP, Schmid CH, et al (1996). "Large trials vs meta-analysis of smaller trials: how do their results compare?". JAMA 276 (16): 1332–8. PMID 8861993. [e]
  6. ↑ Ioannidis JP, Cappelleri JC, Lau J (April 1998). "Issues in comparisons between meta-analyses and large trials". JAMA 279 (14): 1089–93. PMID 9546568. [e]
  7. ↑ 7.0 7.1 The Cochrane Collaboration. Cochrane Handbook
  8. ↑ Royle P, Milne R (2003). "Literature searching for randomized controlled trials used in Cochrane reviews: rapid versus exhaustive searches.". Int J Technol Assess Health Care 19 (4): 591-603. PMID 15095765.
  9. ↑ Stevinson C, Lawlor DA (2004). "Searching multiple databases for systematic reviews: added value or diminishing returns?". Complement Ther Med 12 (4): 228-32. DOI:10.1016/j.ctim.2004.09.003. PMID 15649836. Research Blogging.
  10. ↑ Fergusson D, Laupacis A, Salmi LR, McAlister FA, Huet C (2000). "What should be included in meta-analyses? An exploration of methodological issues using the ISPOT meta-analyses.". Int J Technol Assess Health Care 16 (4): 1109-19. PMID 11155831.
  11. ↑ Hopewell S, Clarke M, Lefebvre C, Scherer R (2007). "Handsearching versus electronic searching to identify reports of randomized trials.". Cochrane Database Syst Rev (2): MR000001. DOI:10.1002/14651858.MR000001.pub2. PMID 17443625. Research Blogging.
  12. ↑ Lemeshow AR, Blum RE, Berlin JA, Stoto MA, Colditz GA (2005). "Searching one or two databases was insufficient for meta-analysis of observational studies.". J Clin Epidemiol 58 (9): 867-73. DOI:10.1016/j.jclinepi.2005.03.004. PMID 16085190. Research Blogging.
  13. ↑ Avenell A, Handoll HH, Grant AM (2001). "Lessons for search strategies from a systematic review, in The Cochrane Library, of nutritional supplementation trials in patients after hip fracture.". Am J Clin Nutr 73 (3): 505-10. PMID 11237924.
  14. ↑ Crumley ET, Wiebe N, Cramer K, Klassen TP, Hartling L (2005). "Which resources should be used to identify RCT/CCTs for systematic reviews: a systematic review.". BMC Med Res Methodol 5: 24. DOI:10.1186/1471-2288-5-24. PMID 16092960. PMC PMC1232852. Research Blogging.
  15. ↑ Wilkins T, Gillies RA, Davies K (2005). "EMBASE versus MEDLINE for family medicine searches: can MEDLINE searches find the forest or a tree?". Can Fam Physician 51: 848-9. PMID 16926954. PMC PMC1479531.
  16. ↑ Savoie I, Helmer D, Green CJ, Kazanjian A (2003). "Beyond Medline: reducing bias through extended systematic review search.". Int J Technol Assess Health Care 19 (1): 168-78. PMID 12701949.
  17. ↑ Whiting P, Westwood M, Burke M, Sterne J, Glanville J (2008). "Systematic reviews of test accuracy should search a range of databases to identify primary studies.". J Clin Epidemiol 61 (4): 357-364. DOI:10.1016/j.jclinepi.2007.05.013. PMID 18313560. Research Blogging.
  18. ↑ Hopewell S, McDonald S, Clarke M, Egger M (2007). "Grey literature in meta-analyses of randomized trials of health care interventions.". Cochrane Database Syst Rev (2): MR000010. DOI:10.1002/14651858.MR000010.pub3. PMID 17443631. Research Blogging.
  19. ↑ McAuley L, Pham B, Tugwell P, Moher D (2000). "Does the inclusion of grey literature influence estimates of intervention effectiveness reported in meta-analyses?". Lancet 356 (9237): 1228-31. DOI:10.1016/S0140-6736(00)02786-0. PMID 11072941. Research Blogging.
  20. ↑ Sampson M, McGowan J, Tetzlaff J, Cogo E, Moher D (2008). "No consensus exists on search reporting methods for systematic reviews.". J Clin Epidemiol 61 (8): 748-54. DOI:10.1016/j.jclinepi.2007.10.009. PMID 18586178. Research Blogging.
  21. ↑ Egger M, Smith GD (1998). "Bias in location and selection of studies.". BMJ 316 (7124): 61-6. PMID 9451274. PMC PMC2665334.
  22. ↑ Cook DJ, Reeve BK, Guyatt GH, Heyland DK, Griffith LE, Buckingham L et al. (1996 Jan 24-31). "Stress ulcer prophylaxis in critically ill patients. Resolving discordant meta-analyses.". JAMA 275 (4): 308-14. PMID 8544272.
  23. ↑ Goodman SN (2002). "The mammography dilemma: a crisis for evidence-based medicine?". Ann Intern Med 137 (5 Part 1): 363-5. PMID 12204023.
  24. ↑ Slavin RE (January 1995). "Best evidence synthesis: an intelligent alternative to meta-analysis". J Clin Epidemiol 48 (1): 9–18. PMID 7853053. [e]
  25. ↑ Higgins JPT, Green S (editors). Table 8.5.a: The Cochrane Collaboration's Tool for assessing risk of bias. Cochrane Handbook for Systematic Reviews of Interventions Version 5.0.1 [updated September 2008]. The Cochrane Collaboration, 2008. Available from www.cochrane-handbook.org.
  26. ↑ Jadad AR, Moore RA, Carroll D, et al (1996). "Assessing the quality of reports of randomized clinical trials: is blinding necessary?". Control Clin Trials 17 (1): 1–12. DOI:10.1016/0197-2456(95)00134-4. PMID 8721797. Research Blogging.
  27. ↑ DerSimonian R, Laird N (1986). "Meta-analysis in clinical trials.". Control Clin Trials 7 (3): 177-88. DOI:10.1016/0197-2456(86)90046-2. PMID 3802833. Research Blogging.
  28. ↑ 28.0 28.1 Diamond GA, Bax L, Kaul S (October 2007). "Uncertain effects of rosiglitazone on the risk for myocardial infarction and cardiovascular death". Ann. Intern. Med. 147 (8): 578–81. PMID 17679700. [e]
  29. ↑ Friedrich JO, Adhikari NK, Beyene J (2007). "Inclusion of zero total event trials in meta-analyses maintains analytic consistency and incorporates all available data.". BMC Med Res Methodol 7: 5. DOI:10.1186/1471-2288-7-5. PMID 17244367. PMC PMC1783664. Research Blogging.
  30. ↑ Bradburn MJ, Deeks JJ, Berlin JA, Russell Localio A (2007). "Much ado about nothing: a comparison of the performance of meta-analytical methods with rare events.". Stat Med 26 (1): 53-77. DOI:10.1002/sim.2528. PMID 16596572. Research Blogging.
  31. ↑ Sweeting MJ, Sutton AJ, Lambert PC (2004). "What to add to nothing? Use and avoidance of continuity corrections in meta-analysis of sparse data.". Stat Med 23 (9): 1351-75. DOI:10.1002/sim.1761. PMID 15116347. Research Blogging.
  32. ↑ Gerstein HC, Miller ME, Byington RP, et al (June 2008). "Effects of intensive glucose lowering in type 2 diabetes". N. Engl. J. Med. 358 (24): 2545–59. DOI:10.1056/NEJMoa0802743. PMID 18539917. Research Blogging.
  33. ↑ 33.0 33.1 33.2 Higgins JP, Thompson SG, Deeks JJ, Altman DG (September 2003). "Measuring inconsistency in meta-analyses". BMJ 327 (7414): 557–60. DOI:10.1136/bmj.327.7414.557. PMID 12958120. PMC 192859. Research Blogging.
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  36. ↑ Higgins JPT, Green S:Cochrane Handbook for Systematic Reviews of Interventions. Cochrane Collaboration (2008). Retrieved on 2008-10-23.
  37. ↑ Altman DG, Bland JM (2003). "Interaction revisited: the difference between two estimates.". BMJ 326 (7382): 219. PMID 12543843. PMC PMC1125071.
  38. ↑ Lau J, Antman EM, Jimenez-Silva J, Kupelnick B, Mosteller F, Chalmers TC (July 1992). "Cumulative meta-analysis of therapeutic trials for myocardial infarction". N. Engl. J. Med. 327 (4): 248–54. PMID 1614465. [e]
  39. ↑ Antman EM, Lau J, Kupelnick B, Mosteller F, Chalmers TC (July 1992). "A comparison of results of meta-analyses of randomized control trials and recommendations of clinical experts. Treatments for myocardial infarction". JAMA 268 (2): 240–8. PMID 1535110. [e]
  40. ↑ Sud S, Douketis J (July 2009). "ACP Journal Club. The devil is in the details...or not? A primer on individual patient data meta-analysis". Ann. Intern. Med. 151 (2): JC1–2, JC1–3. PMID 19620147. [e]
  41. ↑ 41.0 41.1 Poynard T, Munteanu M, Ratziu V, et al (June 2002). "Truth survival in clinical research: an evidence-based requiem?". Ann. Intern. Med. 136 (12): 888–95. PMID 12069563. [e] Cite error: Invalid <ref> tag; name "pmid12069563" defined multiple times with different content
  42. ↑ Thompson SG, Sharp SJ (1999). "Explaining heterogeneity in meta-analysis: a comparison of methods.". Stat Med 18 (20): 2693-708. PMID 10521860.
  43. ↑ Thompson SG, Higgins JP (2002). "How should meta-regression analyses be undertaken and interpreted?". Stat Med 21 (11): 1559-73. DOI:10.1002/sim.1187. PMID 12111920. Research Blogging.
  44. ↑ van Houwelingen HC, Arends LR, Stijnen T (2002). "Advanced methods in meta-analysis: multivariate approach and meta-regression.". Stat Med 21 (4): 589-624. PMID 11836738.
  45. ↑ Jackson D, White IR, Thompson SG (2009). "Extending DerSimonian and Laird's methodology to perform multivariate random effects meta-analyses.". Stat Med. DOI:10.1002/sim.3602. PMID 19408255. Research Blogging.
  46. ↑ Lumley,T. (2009) rmeta: Meta-analysis
  47. ↑ Everitt B, Hothorn, T. (2009) HSAUR2
  48. ↑ A Handbook of Statistical Analyses Using R, Second Edition. Chapman Hall/CRC. ISBN 1-4200-7933-6. 
  49. ↑ Lumley T (August 2002). "Network meta-analysis for indirect treatment comparisons". Stat Med 21 (16): 2313–24. DOI:10.1002/sim.1201. PMID 12210616. Research Blogging.
  50. ↑ Lu G, Ades AE. Combination of direct and indirect evidence in mixed treatment comparisons. Stat Med. 2004 Oct 30;23(20):3105-24. PMID 15449338
  51. ↑ Salanti G, Kavvoura FK, Ioannidis JP (April 2008). "Exploring the geometry of treatment networks". Ann. Intern. Med. 148 (7): 544–53. PMID 18378949. [e]
  52. ↑ Song F, Loke YK, Walsh T, Glenny AM, Eastwood AJ, Altman DG (2009). "Methodological problems in the use of indirect comparisons for evaluating healthcare interventions: survey of published systematic reviews". BMJ 338: b1147. PMID 19346285. PMC 2665205. [e]
  53. ↑ Kent DM, Thaler DE (September 2008). "Stroke prevention--insights from incoherence". N. Engl. J. Med. 359 (12): 1287–9. DOI:10.1056/NEJMe0806806. PMID 18753641. Research Blogging.
  54. ↑ Thijs V, Lemmens R, Fieuws S. Network meta-analysis: simultaneous meta-analysis of common antiplatelet regimens after transient ischaemic attack or stroke. ur Heart J. 2008 May;29(9):1086-92. Epub 2008 Mar 17. PMID 18349026
  55. ↑ Chou, Roger; Susan Carson, Benjamin Chan (2009-02-01). "Gabapentin Versus Tricyclic Antidepressants for Diabetic Neuropathy and Post-Herpetic Neuralgia: Discrepancies Between Direct and Indirect Meta-Analyses of Randomized Controlled Trials". Journal of General Internal Medicine 24 (2): 178-188. DOI:10.1007/s11606-008-0877-5. PMID 19089502. Retrieved on 2009-01-26. Research Blogging.
  56. ↑ Olsen O, Middleton P, Ezzo J, et al (2001). "Quality of Cochrane reviews: assessment of sample from 1998". BMJ 323 (7317): 829–32. PMID 11597965. [e]
  57. ↑ Yank V, Rennie D, Bero LA (2007). "Financial ties and concordance between results and conclusions in meta-analyses: retrospective cohort study.". BMJ 335 (7631): 1202-5. DOI:10.1136/bmj.39376.447211.BE. PMID 18024482. PMC PMC2128658. Research Blogging.
  58. ↑ Sutton AJ, Duval SJ, Tweedie RL, Abrams KR, Jones DR (2000). "Empirical assessment of effect of publication bias on meta-analyses.". BMJ 320 (7249): 1574-7. PMID 10845965. PMC PMC27401.
  59. ↑ 59.0 59.1 Egger M, Davey Smith G, Schneider M, Minder C (1997). "Bias in meta-analysis detected by a simple, graphical test.". BMJ 315 (7109): 629-34. PMID 9310563. PMC PMC2127453.
  60. ↑ Pham B, Platt R, McAuley L, Klassen TP, Moher D (2001). "Is there a "best" way to detect and minimize publication bias? An empirical evaluation.". Eval Health Prof 24 (2): 109-25. PMID 11523382.
  61. ↑ Terrin N et al. (2005). "In an empirical evaluation of the funnel plot, researchers could not visually identify publication bias". J Clin Epidemiol 58: 894–901. DOI:10.1016/j.jclinepi.2005.01.006. PMID 16085192. Research Blogging.
  62. ↑ Furukawa TA, Watanabe N, Omori IM, Montori VM, Guyatt GH (February 2007). "Association between unreported outcomes and effect size estimates in Cochrane meta-analyses". JAMA 297 (5): 468–70. DOI:10.1001/jama.297.5.468-b. PMID 17284696. Research Blogging.
  63. ↑ Chan AW, Hróbjartsson A, Haahr MT, Gøtzsche PC, Altman DG (May 2004). "Empirical evidence for selective reporting of outcomes in randomized trials: comparison of protocols to published articles". JAMA 291 (20): 2457–65. DOI:10.1001/jama.291.20.2457. PMID 15161896. Research Blogging.
  64. ↑ Shojania KG, Sampson M, Ansari MT, Ji J, Doucette S, Moher D (August 2007). "How quickly do systematic reviews go out of date? A survival analysis". Ann. Intern. Med. 147 (4): 224–33. PMID 17638714. [e]
  65. ↑ The use of older studies in meta-analyses of medical interventions: a survey (Text.Serial.Journal) (2009-05-11). Retrieved on 2009-06-04.
  66. ↑ Sampson M, Shojania KG, McGowan J, et al (August 2008). "Surveillance search techniques identified the need to update systematic reviews". J Clin Epidemiol 61 (8): 755–62. DOI:10.1016/j.jclinepi.2007.10.003. PMID 18586179. Research Blogging.

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