Conn Vicki S, Valentine Jeffrey C, Cooper Harris M, Rantz Marilyn J
Department of Psychological Sciences, University of Missouri-Columbia, USA.
Nurs Res. 2003 Jul-Aug;52(4):256-61. doi: 10.1097/00006199-200307000-00008.
In meta-analysis, researchers combine the results of individual studies to arrive at cumulative conclusions. Meta-analysts sometimes include "grey literature" in their evidential base, which includes unpublished studies and studies published outside widely available journals. Because grey literature is a source of data that might not employ peer review, critics have questioned the validity of its data and the results of meta-analyses that include it.
To examine evidence regarding whether grey literature should be included in meta-analyses and strategies to manage grey literature in quantitative synthesis.
This article reviews evidence on whether the results of studies published in peer-reviewed journals are representative of results from broader samplings of research on a topic as a rationale for inclusion of grey literature. Strategies to enhance access to grey literature are addressed.
The most consistent and robust difference between published and grey literature is that published research is more likely to contain results that are statistically significant. Effect size estimates of published research are about one-third larger than those of unpublished studies. Unfunded and small sample studies are less likely to be published. Yet, importantly, methodological rigor does not differ between published and grey literature.
Meta-analyses that exclude grey literature likely (a) over-represent studies with statistically significant findings, (b) inflate effect size estimates, and (c) provide less precise effect size estimates than meta-analyses including grey literature. Meta-analyses should include grey literature to fully reflect the existing evidential base and should assess the impact of methodological variations through moderator analysis.
在荟萃分析中，研究人员综合各项独立研究的结果以得出累积结论。荟萃分析者有时会将“灰色文献”纳入其证据基础，灰色文献包括未发表的研究以及在广泛可得期刊之外发表的研究。由于灰色文献可能并非经过同行评审的数据来源，批评者对其数据的有效性以及包含灰色文献的荟萃分析结果提出了质疑。
探讨关于灰色文献是否应纳入荟萃分析的证据以及在定量综合中管理灰色文献的策略。
本文回顾了关于同行评审期刊上发表的研究结果是否代表某一主题更广泛研究样本结果的证据，以此作为纳入灰色文献的理由。文中还讨论了增加获取灰色文献途径的策略。
已发表文献与灰色文献之间最一致且显著的差异在于，已发表的研究更有可能包含具有统计学显著性的结果。已发表研究的效应量估计值比未发表研究大约大三分之一。无资金支持和小样本研究发表的可能性较小。然而，重要的是，已发表文献和灰色文献在方法严谨性方面并无差异。
排除灰色文献的荟萃分析可能（a）过度代表具有统计学显著结果的研究，（b）夸大效应量估计值，并且（c）与纳入灰色文献的荟萃分析相比，提供的效应量估计值不够精确。荟萃分析应纳入灰色文献以充分反映现有证据基础，并应通过调节分析评估方法学差异的影响。
