Methodology Review

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

  1. Sally Hopewell1,*,
  2. Steve McDonald2,
  3. Mike J Clarke1,
  4. Matthias Egger3

Editorial Group: Cochrane Methodology Review Group

Published Online: 18 APR 2007

Assessed as up-to-date: 19 FEB 2007

DOI: 10.1002/14651858.MR000010.pub3


How to Cite

Hopewell S, McDonald S, Clarke MJ, Egger M. Grey literature in meta-analyses of randomized trials of health care interventions. Cochrane Database of Systematic Reviews 2007, Issue 2. Art. No.: MR000010. DOI: 10.1002/14651858.MR000010.pub3.

Author Information

  1. 1

    UK Cochrane Centre, Oxford, Oxfordshire, UK

  2. 2

    Monash Institute of Health Services Research, Australasian Cochrane Centre, Clayton, Victoria, Australia

  3. 3

    Institute of Social and Preventive Medicine, Department of Social Medicine, Bern, Switzerland

*Sally Hopewell, UK Cochrane Centre, National Institute for Health Research, Summertown Pavilion, Middle Way, Oxford, Oxfordshire, OX2 7LG, UK. shopewell@cochrane.co.uk.

Publication History

  1. Publication Status: Edited (no change to conclusions)
  2. Published Online: 18 APR 2007

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Abstract

  1. Top of page
  2. Abstract
  3. Plain language summary
  4. 摘要

Background

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.

Objectives

To review systematically research studies, which have investigated the impact of grey literature in meta-analyses of randomized trials of health care interventions.

Search methods

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.

Selection criteria

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 collection and analysis

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.

Main results

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.

Authors' conclusions

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.

 

Plain language summary

  1. Top of page
  2. Abstract
  3. Plain language summary
  4. 摘要

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

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.

 

摘要

  1. Top of page
  2. Abstract
  3. Plain language summary
  4. 摘要

背景

針對臨床處置的隨機臨床試驗之統合分析研究﹐回顧灰色文獻(grey literature)的影響

在系統性回顧時納入灰色文獻(即非正式發表的文獻)可能有助於克服出版偏差,後者乃因資料的可取得性差異所致。

目標

凡探索灰色文獻對隨機臨床試驗統合分析結果之影響的相關研究,收集並進行系統性的回顧。

搜尋策略

我們搜尋了Cochrane Methodology Register (The Cochrane Library Issue 3, 2005), MEDLINE (1966年到2005年5月20日), the Science Citation Index (2005年6月),並聯絡相關研究人士。

選擇標準

凡研究目的為比較1) 納入灰色文獻或2) 排除灰色文獻對隨機臨床試驗統合分析結果之影響的研究﹐均納入本次系統回顧。

資料收集與分析

兩位作者各自獨立從報告中擷取資料。主要的結果指標是灰色文獻中的臨床試驗對統合分析綜合估計的影響,同時收集以下資料:健康照顧的領域、統合分析的數目、臨床試驗的數目、參與臨床試驗的人數、臨床試驗出版的年份、出版的國家及語言、灰色文獻及出版文獻的種類及數目﹐及研究方法的品質。

主要結論

有五篇研究符合入選條件。這五篇一致顯示已出版的臨床試驗的療效比灰色文獻的大。其中一篇的差異具統計上顯著意義。其中有三篇的資料可以合併﹐顯示已出版的臨床試驗其療效比灰色文獻的療效大9%(已出版的臨床試驗對灰色文獻的odds ratios 的比例為1.09; 95% CI 1.03 – 1.16)。統合分析納入的臨床試驗已出版的多於灰色文獻(median 224 (IQR 108 – 365) 對 45(IQR 40 – 102))。已出版的臨床試驗平均受試者較多。灰色文獻最常見的型式為摘要(55%)及未出版的資料(30%)。關於灰色文獻在方法學上的品質是否較差﹐沒有足夠的證據。

作者結論

本回顧顯示﹐已出版的臨床試驗其規模及整體療效較灰色文獻為大。其重要意涵為:文獻回顧者應確實找出灰色文獻一併回顧,以降低引進偏差的風險至最低。

翻譯人

本摘要由慈濟醫院葉日弌翻譯。

此翻譯計畫由臺灣國家衛生研究院(National Health Research Institutes, Taiwan)統籌。

總結

本篇方法學回顧找到5篇探討系統性回顧醫療處置的灰色文獻的效應。結果顯示﹐已出版的臨床試驗規模及效果均大於灰色文獻。關於灰色文獻在方法學上的品質是否較差的證據有限。其意涵為:文獻回顧應包含已出版的及灰色文獻,以將出版偏差的影響減少到最低的程度。