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 The most common types of grey literature were abstracts (55%) and unpublished data (30%).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.
"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. "
[Show abstract][Hide abstract] 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.
Frontiers in Psychology 07/2015; 6:1105. DOI:10.3389/fpsyg.2015.01105 · 2.80 Impact Factor
"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). "
[Show abstract][Hide abstract] 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.
Journal of Youth and Adolescence 02/2015; 44(3):616-636. DOI:10.1007/s10964-014-0226-4 · 2.72 Impact Factor
"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
, and can therefore be retrieved via search engines i.e. Google and Google Scholar. "
[Show abstract][Hide abstract] ABSTRACT: Background
Sedation in palliative care has received growing attention in recent years; and so have guidelines, position statements, and related literature that provide recommendations for its practice. Yet little is known collectively about the content, scope and methodological quality of these materials.
According to research, there are large variations in palliative sedation practice, depending on the definition and methodology used. However, a standardised approach to comparing and contrasting related documents, across countries, associations and governmental bodies is lacking. This paper reports on a protocol designed to enable thorough and systematic comparison of guidelines and guidance documents on palliative sedation.
Methods and design
A multidisciplinary and international group of palliative care researchers, identified themes and clinical issues on palliative sedation based on expert consultations and evidence drawn from the EAPC (European Association of Palliative Care) framework for palliative sedation and AGREE II (Appraisal Guideline Research and Evaluation) instrument for guideline assessment. The most relevant themes were selected and built into a comprehensive checklist. This was tested on people working closely with practitioners and patients, for user-friendliness and comprehensibility, and modified where necessary. Next, a systematic search was conducted for guidelines in English, Dutch, Flemish, or Italian. The search was performed in multiple databases (PubMed, CancerLit, CNAHL, Cochrane Library, NHS Evidence and Google Scholar), and via other Internet resources. Hereafter, the final version of the checklist will be used to extract data from selected literature, and the same will be compiled, entered into SPSS, cleaned and analysed systematically for publication.
We have together developed a comprehensive checklist in a scientifically rigorous manner to allow standardised and systematic comparison. The protocol is applicable to all guidelines on palliative sedation, and the approach will contribute to rigorous and systematic comparison of international guidelines on any challenging topic such as this. Results from the study will provide valuable insights into common core elements and differences between the selected guidelines, and the extent to which recommendations are derived from, or match those in the EAPC framework. The outcomes of the study will be disseminated via peer-reviewed journals and directly to appropriate audiences.
BMC Palliative Care 07/2014; 13(1):34. DOI:10.1186/1472-684X-13-34 · 1.78 Impact Factor
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