Publication bias

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In academic journals and scientific journals, publication bias is defined as "the influence of study results on the chances of publication and the tendency of investigators, reviewers, and editors to submit or accept manuscripts for publication based on the direction or strength of the study findings. Publication bias has an impact on the interpretation of clinical trials and meta-analyses. Bias can be minimized by insistence by editors on high-quality research, thorough literature reviews, acknowledgment of conflicts of interest, modification of peer review practices, etc."[1]

Publication bias has been documented to occur.[2]

The small study effect is closely related. The small study effect is the observation that small studies tend to report more positive results.[3][4] This is especially a threat when the original studies in a meta-analysis are less than 50 patients in size.[5]


An example of probable publication bias is in the studies of glucosamine and chondroitin for treatment of osteoarthritis. In an initial meta-analysis, the authors noted evidence of publication bias during examination of the results.[6]

A subsequent large randomized controlled trial[7] and meta-analyses including the large trial were negative.[8][9]

Another example is the selective publication of randomized controlled trials of antidepressants[10] or of positive trials in general[11].

Anecdotal examples include partial publication of results of trails of the antiviral agent oseltamivir for the prevention of influenza.[12]

Frequency of occurrence

Randomized controlled trials

Publication bias, or bias in interpretation of studies, may occur in 25%[13] to 60% of some types of articles.[14][15][16] Publication bias may be associated with the language the study is published in and may be more common in certain areas of study. In the field of complementary and alternative medicine (CAM), publication bias may be more likely to occur in studies published in languages other than English.[17]

A possible reason is that typical CAM studies are smaller than trials of conventional medicine.[17] According to two European analyses, both including German authors and one from the German Cochrane Center, this bias may be especially prevalent in German publications.[18][19]

Negative studies are less likely to be published[20][21] and have more delays in their publication[22].

Studies of diagnostic tests

Publication bias may occur in studies of diagnostic tests.[23] Publication bias may be more of a problem in diagnostic test research than in randomized controlled trials because studies of diagnostic tests can be secondary analyses of databases and do not have to be registered prior to publication.[24]


In meta-analyses of randomized controlled trials, a file drawer[25], a funnel plot analysis[26][27], or a contour enhanced funnel plot[28] may help detect underlying publication bias among the studies in the meta-analysis. However, other factors may cause asymmetry[29] and the interpretation of the graph is subjective and inconsistent[27].

In meta-analysis of diagnostic tests, the effective sample size funnel plot and associated regression test of asymmetry may be used.[30]


Many factors influence publication.[2][31] Publication bias may be due to authors not submitting negative studies for publication.[31] This may especially be true of studies authored by industries with conflicts of interest.[32][33] Phase I studies may be more susceptible.[34] Studies sponsored by industry are more likely to be positive.[35][36]

It is not simple to get a study published in any peer-reviewed journal, least of all in the best journals. Accordingly, many studies go unreported. It is often thought to be difficult to publish small studies, the outcome of which conflicts with the reported outcomes of larger previously published studies, or to publish studies from which no clear conclusion can be drawn. In part, this reflects the wish of the best journals to publish influential papers, and in part it reflects authors choosing not to publish studies that are thought to be uninteresting. Such publication bias can be difficult to recognise, but its effects tend to encourage publication of studies that support an already formed conclusion, while discouraging publication of contradictory or equivocal findings.[2][31]

Related biases

Language bias

Language bias may affect the results of meta-analyses.[37]

Availability of full text on the Internet

e-publication bias

Stuides sponsored by industry may be more likely to be published with open access by journals with hybrid publication models.[38]

Full Text On the Net (FUTON) bias

Full Text On the Net (FUTON) bias may affect results of information retrieval.[39]

No abstract available (NAA) bias

Certain knowledge domains may be more susceptible to 'no abstract available' (NAA) bias.[39]

Selective outcome reporting bias

Among published articles, analyses of outcomes reported to be studied in unpublished protocols[40][16], FDA submissions[41], trial registration[42], or the methods sections of published papers[43] compared to the outcomes actually reported in the results of published papers found that outcomes with significant p-values are more likely to be reported, and reported sufficiently to allow inclusion in meta-analysis, than insignificant outcomes.

Selective outcome reporting bias may cause results of meta-analyses to be exaggerated.[44]

Impact of publication bias

Impact on meta-analysis

Publication bias is a major threat to the validity of meta-analysis. Publication bias against negative studies may threaten the validity of meta-analyses that are positive and all the studies included within the meta-analysis are small.[45][26]

In performing a meta-analyses, a file drawer[25] or a funnel plot analysis[26][27] may help detect underlying publication bias among the studies in the meta-analysis.

Publication bias has also occurred in the publication of randomized controlled trials of antidepressants.[10]

Impact on the drug approval process

Publication bias occurs in the data submitted to the United States Food and Drug Administration[46][47] and to the Swedish drug regulatory authority[32]by drug companies.

Impact on media coverage of scientific research

In journalism, the media is more likely to cover studies that report positive results.[48]


Trial registration

In 2004, the International Committee of Medical Journal Editors (ICMJE) announced that all trials starting enrollment after July 1, 2005 must be registered prior to consideration for publication in one of the 12 member journals of the Committee.[49] Randomized controlled trials may be registered at sites such as,, and


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