Article

Searching for unpublished trials in Cochrane reviews may not be worth the effort

Department of General Practice and Primary Health Care, Ghent University, Belgium.
Journal of clinical epidemiology (Impact Factor: 5.48). 08/2009; 62(8):838-844.e3. DOI: 10.1016/j.jclinepi.2008.09.010
Source: PubMed

ABSTRACT To assess the value of searching for unpublished data by exploring the extent to which Cochrane reviews include unpublished data and by evaluating the quality of unpublished trials.
We screened all 2,462 completed Cochrane reviews published since 2000 in the Cochrane Database of Systematic Reviews Issue 3, 2006. In a random sample (n=61) of 292 reviews, including unpublished trials, we studied all 116 references.
Unpublished trials make up 8.8% of all included trials in our sample. Thirty-eight percent of the "unpublished" trials have in fact been published. Allocation concealment was "unclear" or not adequate in 54.3% and 61.3% reported blinding. In 47.2% reported withdrawal rates were >20%. Trials that were eventually published had larger mean population sizes (P-value, 0.02). Of the reported sponsors, 87.3% were drug companies. Methodological quality and publication bias are mentioned in half of the reviews and explored in a third. Quality ratings did not have consequences for pooling, because 82.8% was included in the forest plots.
A minority of Cochrane reviews include "unpublished trials" and many of these are eventually published. Truly unpublished studies have poor or unclear methodological quality. Therefore, it may be better to invest in regular updating of reviews, rather than in extensive searching for unpublished data.

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Available from: Jan De Maeseneer, Aug 29, 2015
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    • "It has been suggested that identifying non- English language and unpublished studies for inclusion is important to minimize language and publication bias respectively. However, this has been questioned and suggested that many unpublished studies eventually become published and truly unpublished studies might have poor or unclear methodology, which in turn might serve to introduce bias to any systematic review (Van Driel et al., 2009). "
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    • "While there is little doubt in the investigators’ minds regarding the latter recommendation, avoiding the use of publication status as an inclusion criterion could be questioned. For example, van Driel et al. suggested that (1) the difficulty in retrieving unpublished work could lead to selection bias, (2) many unpublished trials are eventually published, (3) the methodological quality of such studies are poorer than those that are published, and (4) the effort and resources required to obtain unpublished work may not be warranted [46]. "
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    • "The former notwithstanding, avoiding the use of publication status as an inclusion criterion could be questioned. For example, van Driel et al. [46] concluded that (1) the difficulty in retrieving unpublished work could lead to selection bias, (2) many unpublished trials are eventually published, (3) the methodological quality of such studies is poorer than those that are published, and (4) the effort and resources required to obtain unpublished work may not be warranted. "
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