Grey Literature in meta-analyses of randomized trials of health care interventions

UK Cochrane Centre, NHS R&D Programme, Summertown Pavilion, Middle Way, Oxford, Oxfordshire, UK, OX2 7LG.
Cochrane database of systematic reviews (Online) (Impact Factor: 6.03). 02/2007; 2(2):MR000010. DOI: 10.1002/14651858.MR000010.pub3
Source: PubMed


The inclusion of grey literature (i.e. literature that has not been formally published) in systematic reviews may help to overcome some of the problems of publication bias, which can arise due to the selective availability of data.
To review systematically research studies, which have investigated the impact of grey literature in meta-analyses of randomized trials of health care interventions.
We searched the Cochrane Methodology Register (The Cochrane Library Issue 3, 2005), MEDLINE (1966 to 20 May 2005), the Science Citation Index (June 2005) and contacted researchers who may have carried out relevant studies.
A study was considered eligible for this review if it compared the effect of the inclusion and exclusion of grey literature on the results of a cohort of meta-analyses of randomized trials.
Data were extracted from each report independently by two reviewers. The main outcome measure was an estimate of the impact of trials from the grey literature on the pooled effect estimates of the meta-analyses. Information was also collected on the area of health care, the number of meta-analyses, the number of trials, the number of trial participants, the year of publication of the trials, the language and country of publication of the trials, the number and type of grey and published literature, and methodological quality.
Five studies met the inclusion criteria. All five studies showed that published trials showed an overall greater treatment effect than grey trials. This difference was statistically significant in one of the five studies. Data could be combined for three of the five studies. This showed that, on average, published trials showed a 9% greater treatment effect than grey trials (ratio of odds ratios for grey versus published trials 1.09; 95% CI 1.03-1.16). Overall there were more published trials included in the meta-analyses than grey trials (median 224 (IQR 108-365) versus 45(IQR 40-102)). Published trials had more participants on average. The most common types of grey literature were abstracts (55%) and unpublished data (30%). There is limited evidence to show whether grey trials are of poorer methodological quality than published trials.
This review shows that published trials tend to be larger and show an overall greater treatment effect than grey trials. This has important implications for reviewers who need to ensure they identify grey trials, in order to minimise the risk of introducing bias into their review.

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    • "Studies, with human participants, that were published in English were included. Theses, conference proceedings and unpublished studies were also included because its inclusion could minimize the risk of bias (Hopewell et al., 2007). No publication date restriction was applied. "
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    Frontiers in Psychology 07/2015; 6:1105. DOI:10.3389/fpsyg.2015.01105 · 2.80 Impact Factor
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    • "To limit the risk of publication bias, locating unpublished literature for inclusion in systematic review is a crucial component of the search strategy as outlined by the Campbell Collaboration (2014). Although unpublished studies, such as dissertations, have not gone through the formal peer-review process, unpublished and published research have been found to be of similar quality (McLeod and Weisz 2004; Hopewell et al. 2007; Moyer et al. 2010). "
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    ABSTRACT: The popularity, demand, and increased federal and private funding for after-school programs have resulted in a marked increase in after-school programs over the past two decades. After-school programs are used to prevent adverse outcomes, decrease risks, or improve functioning with at-risk youth in several areas, including academic achievement, crime and behavioral problems, socio-emo-tional functioning, and school engagement and attendance; however, the evidence of effects of after-school programs remains equivocal. This systematic review and meta-ana-lysis, following Campbell Collaboration guidelines, examined the effects of after-school programs on exter-nalizing behaviors and school attendance with at-risk stu-dents. A systematic search for published and unpublished literature resulted in the inclusion of 24 studies. A total of 64 effect sizes (16 for attendance outcomes; 49 for exter-nalizing behavior outcomes) extracted from 31 reports were included in the meta-analysis using robust variance estimation to handle dependencies among effect sizes. Mean effects were small and non-significant for attendance and externalizing behaviors. A moderate to large amount of heterogeneity was present; however, no moderator variable tested explained the variance between studies. Significant methodological shortcomings were identified across the corpus of studies included in this review. Implications for practice, policy and research are discussed.
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    • "Rather, they are produced with the end-users in mind. Accessing such literature that has not been formally published (also called grey literature) can be problematic despite the fact that some of these documents, produced by government and non-government institutions, are increasingly published on the internet [34], and can therefore be retrieved via search engines i.e. Google and Google Scholar. "
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