In 1989, Gøtzsche found that, among 244 reports of trials comparing non-steroidal anti-inflammatory drugs in rheumatoid arthritis, 44 (18%) were redundant, multiple publications, which overlapped substantially with a previously published article. Twenty trials were published twice, ten trials three times and one trial four times (Gøtzsche 1989). The production of multiple publications from single studies can lead to bias in a number of ways (Huston 1996). Most importantly, studies with significant results are more likely to lead to multiple publications and presentations (Easterbrook 1991), which makes it more likely that they will be located and included in a meta-analysis. It is not always obvious that multiple publications come from a single study, and one set of study participants may be included in an analysis twice. The inclusion of duplicated data may therefore lead to overestimation of intervention effects, as was demonstrated for trials of the efficacy of ondansetron to prevent postoperative nausea and vomiting (Tramèr 1997).
Other authors have described the difficulties and frustration caused by redundancy and the ‘disaggregation’ of medical research when results from a multi-centre trial are presented in several publications (Huston 1996, Johansen 1999). Redundant publications often fail to cross-reference each other (Bailey 2002, Barden 2003) and there are examples where two articles reporting the same trial do not share a single common author (Gøtzsche 1989, Tramèr 1997). Thus, it may be difficult or impossible for review authors to determine whether two papers represent duplicate publications of one study or two separate studies without contacting the authors, which may result in biasing a meta-analysis of this data.