Grey literature in meta-analyses of randomized trials of health care interventions (Cochrane Methodology Review)

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 1, 2002), MEDLINE (1966 to February 2002), the Science Citation Index (April 2002) 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 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.

Eight studies meeting the inclusion criteria were identified. Four studies contained multiple meta-analyses and four contained single meta-analyses. Of the included studies, four multiple and three single meta-analyses, found that published trials showed an overall greater treatment effect than grey trials. This difference was statistically significant in two of the four multiple meta-analyses. The remaining single meta-analysis found that published trials showed no effect of treatment and that grey trials showed a negative treatment effect; this difference was not statistically significant. Overall there were more published trials included in the meta-analyses than grey trials (median 46 (IQR 4-300) versus 5.5 (IQR 4-88)). Published trials had more participants on average. In the two studies that assessed methodological quality of the included trials, the published trials were of higher quality than the grey trials. The most common types of grey literature were abstracts (49%) and unpublished data (33%).

Authors' conclusions:

This review suggests that published trials are generally larger and may 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.

Citation: Hopewell S, McDonald S, Clarke M, Egger M.. Grey literature in meta-analyses of randomized trials of health care interventions (Cochrane Methodology Review). The Cochrane Database of Methodology Reviews 0000, Issue . Art. No.: MR000010. DOI:10.1002/14651858.MR000010.