On 2014 May 11, Hilda Bastian commented:

An important reminder that multiple publication bias - which can lead to double-counting of patients - in meta-analysis has not disappeared (PMC full text). Choi and colleagues point to a recent study suggesting the incidence of duplicate publication in the field of otolaryngology didn't change over 10 years (Cheung VW, 2014), although it may have reduced in some other fields.

Systematic reviewers weed out most duplicate reporting, but as this new study shows, some still slip through. In a meta-analysis, the magnification of events can tip the balance of evidence. A study a decade ago showed that authorship was an unreliable criterion for detecting duplicate publication of trial data (von Elm E, 2004), and the publications don't cross-reference each other, either. Choi and colleagues don't raise the issue of the importance of clinical trial registration here: Antes and Dickersin pointed to this as a key strategy to address this problem (Antes G, 2004).

There is also duplicate registration of trials in different registers, though (Zarin DA, 2007, Califf RM, 2012). ClinicalTrials.gov aims to identify and resolve duplicate registration of trials (Zarin DA, 2007), and most registered trials are included there. Consistent citation of trial registration numbers, especially the ClinicalTrials.gov identification (NCT number), in all systematic reviews of trials would be useful for readers and those trying to identify studies. It might help reduce reviewers' workload in weeding out duplicate reports, too.

(I work on projects related to systematic reviews at NCBI (National Center for Biotechnology Information, U.S. National Library of Medicine), which is also responsible for ClinicalTrials.gov.)

On 2014 May 11, Hilda Bastian commented:

An important reminder that multiple publication bias - which can lead to double-counting of patients - in meta-analysis has not disappeared (PMC full text). Choi and colleagues point to a recent study suggesting the incidence of duplicate publication in the field of otolaryngology didn't change over 10 years (Cheung VW, 2014), although it may have reduced in some other fields.

Systematic reviewers weed out most duplicate reporting, but as this new study shows, some still slip through. In a meta-analysis, the magnification of events can tip the balance of evidence. A study a decade ago showed that authorship was an unreliable criterion for detecting duplicate publication of trial data (von Elm E, 2004), and the publications don't cross-reference each other, either. Choi and colleagues don't raise the issue of the importance of clinical trial registration here: Antes and Dickersin pointed to this as a key strategy to address this problem (Antes G, 2004).

There is also duplicate registration of trials in different registers, though (Zarin DA, 2007, Califf RM, 2012). ClinicalTrials.gov aims to identify and resolve duplicate registration of trials (Zarin DA, 2007), and most registered trials are included there. Consistent citation of trial registration numbers, especially the ClinicalTrials.gov identification (NCT number), in all systematic reviews of trials would be useful for readers and those trying to identify studies. It might help reduce reviewers' workload in weeding out duplicate reports, too.

(I work on projects related to systematic reviews at NCBI (National Center for Biotechnology Information, U.S. National Library of Medicine), which is also responsible for ClinicalTrials.gov.)