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


This methodology review identified five studies which investigated the effect of including trials found in the grey literature in systematic reviews of health care interventions. They showed that trials found in the published literature tend to be larger and show larger effects of a health care intervention than those trials found in the grey literature. There was limited evidence to show whether grey trials are of poorer methodological quality than published trials. This means that those carrying out systematic reviews need to search for trials in both the published and grey literature in order to help minimise the effects of publication bias in 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 Theses, conference proceedings and unpublished studies were also included because its inclusion could minimize the risk of bias (Hopewell et al., 2007).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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    ABSTRACT: Self-selected exercise seems to promote positive affective responses due to the perceived autonomy associated with it. The objective of the present study was to determine the magnitude of differences in Feeling Scale (FS) responses during self-selected and imposed exercise sessions. The PRISMA Statement was adopted for this meta-analysis. The search used PubMed, Scopus, PsycINFO, and ISI Web of Knowledge databases. A total of 10 studies that compared the effects of self-selected and imposed exercise sessions on acute FS responses were included. The screening strategy included: exclusion of studies that were duplicated between databases, abstract screening, and text screening. The standardized mean difference (SMD) between self-selected and imposed exercise sessions categorized in five intensities (equal intensity: both exercises were performed at the same intensity, below lactate/ventilatory threshold (LT/VT): imposed exercise was performed at an intensity below the LT/VT, at LT/VT: imposed exercise was performed at the LT/VT intensity, above LT/VT: imposed exercise was performed at an intensity above the LT/VT, and different intensity: both exercises were performed at different intensities and the intensity of imposed session was not reported relative to LT/VT) and an overall SMD were calculated. Self-selected exercise was used as the reference condition. The subtotal SMD values were as follows: -.10 (equal intensity), -.36 (below LT/VT), -.57 (at LT/VT), -1.30 (above LT/VT), and -.09 (different intensity). The overall SMD was -.41. These results have shown that the differences in the exercise intensity used in imposed exercise sessions influenced the affective responses considering that higher intensities of imposed session provided higher effect size in favor of self-selected exercise.
    Full-text · Article · Jul 2015 · Frontiers in Psychology
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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.
    Full-text · Article · Feb 2015 · Journal of Youth and Adolescence
    • "4.882007 Notes 1. Although including grey literature may attenuate publication bias, given that published studies generally have larger treatment effects than unpublished studies (Hopewell, McDonald, Clarke, & Egger, 2007), grey literature can also introduce bias due to poorer methodological quality (Egger, Juni, Barlett, Holenstein, & Sterne, 2003) and comprehensiveness of locatable grey literature (Sterne, Egger, & Moher, 2011). Given that reviews that include grey literature produce similar results to those that do not (Egger et al., 2003), it was decided to restrict the search to published findings. "
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    ABSTRACT: Decades of research have suggested that expressive writing produces physical and psychological benefits in controlled laboratory experiments among healthy college students. This work has been extended to clinical and medical populations, including cancer patients. Although expressive writing could be a promising and inexpensive intervention for this population, the effects have not been systematically examined in oncology samples. A systematic review using PRISMA guidelines was conducted for experimental trials of cancer patients who participated in an expressive writing intervention. PsycINFO and PubMed/Medline were searched for peer-reviewed studies. Thirteen articles met the inclusion/exclusion criteria. Although the majority of the intervention effects were null, there were several main effects for expressive writing on sleep, pain, and general physical and psychological symptoms. Several moderators were identified, suggesting that expressive writing may be more or less beneficial based on individual characteristics such as social constraints. The reviewed studies were limited due to representativeness of the samples, performance, detection and patient-reported outcomes biases, and heterogeneity of the intervention protocol and writing prompts. Future studies with rigorous designs are needed to determine whether expressive writing is therapeutically effective in cancer patients.
    No preview · Article · Sep 2014 · Health Psychology Review
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