With the increasing use of meta-analysis, duplicate publication of original research is particularly problematic. Duplicate publication can result in an inappropriate weighting of the study results. The purpose of our study was to assess the incidence and characteristics of duplicate publications in Korea, and to estimate the impact of duplicate publication on meta-analyses. The meta-analysis literature written by Korean authors was searched using the online search engines PubMed, KMbase, and KoreaMed. Duplication patterns were classified into the following 4 combinations: identical samples and identical outcomes (copy), identical samples and different outcomes (fragmentation), increased samples and identical outcomes (imalas), and decreased samples and identical outcomes (disaggregation). To estimate the multiple publication bias, we performed a meta-analysis with and without duplicated data. We estimated that 6 (6.9%) of the 86 analyzed meta-analyses included duplicate publications, and 6 of the 1,194 articles (0.5%) used in the meta-analyses were duplicate publications. In this study, duplicate publications were usually due to disaggregation and overlapping (imalas) publications. Of 6 duplicated articles, 1 was considered a copy (16.6%); 1, a fragmentation (16.6%); 2, imalas (33.3%); and 2, disaggregations (33.3%). There was an increase in the mean effect size and fail-safe number with duplicated data. Our study found only 6 instances of duplicate publication after analyzing 1,194 articles used in meta-analyses written by Korean authors. However, 6.9% of the meta-analyses included duplicate publications. Our findings suggest that meta-analyses should be interpreted cautiously, taking into account the possibility of duplicated studies.
Keywords: Bias; Duplicate publication; Korea; Meta-analysis.