Intended for healthcare professionals


Impact of covert duplicate publication on meta-analysis: a case study

BMJ 1997; 315 doi: (Published 13 September 1997) Cite this as: BMJ 1997;315:635
  1. Martin R Tramèr, research fellow (martin.tramer%mailgate.jr2{at},
  2. D John M Reynolds, consultant physician and clinical pharmacologistb,
  3. R Andrew Moore, consultant biochemista,
  4. Henry J McQuay, clinical reader in pain reliefa
  1. a Pain Research, Nuffield Department of Anaesthetics, Churchill Hospital, Oxford OX3 7LJ
  2. b John Radcliffe Hospital, Oxford OX3 9DU
  1. Correspondence to: Dr Tramèr
  • Accepted 21 July 1997


Objective: To quantify the impact of duplicate data on estimates of efficacy.

Design: Systematic search for published full reports of randomised controlled trials investigating ondansetron's effect on postoperative emesis. Abstracts were not considered.

Data sources: Eighty four trials (11 980 patients receiving ondansetron) published between 1991 and September 1996.

Main outcome measures: Percentage of duplicated trials and patient data. Estimation of antiemetic efficacy (prevention of emesis) of the most duplicated ondansetron regimen. Comparison between the efficacy of non-duplicated and duplicated data.

Results: Data from nine trials had been published in 14 further reports, duplicating data from 3335 patients receiving ondansetron; none used a clear cross reference. Intravenous ondansetron 4 mg versus placebo was investigated in 16 reports not subject to duplicate publication, three reports subject to duplicate publication, and six duplicates of those three reports. The number needed to treat to prevent vomiting within 24 hours was 9.5 (95% confidence interval 6.9 to 15) in the 16 non-duplicated reports and 3.9 (3.3 to 4.8) in the three reports which were duplicated (P<0.00001). When these 19 were combined the number needed to treat was 6.4 (5.3 to 7.9). When all original and duplicate reports were combined (n=25) the apparent number needed to treat improved to 4.9 (4.4 to 5.6).

Conclusions: By searching systematically we found 17% of published full reports of randomised trials and 28% of the patient data were duplicated. Trials reporting greater treatment effect were significantly more likely to be duplicated. Inclusion of duplicated data in meta-analysis led to a 23% overestimation of ondansetron's antiemetic efficacy.

Key messages

  • Although publishing the same data more than once is strongly discouraged, there is no evidence of the impact of duplicate data on meta-analysis

  • Re-analysing an important trial, and cross referencing to original reports (overt duplication), may be necessary and valuable in some circumstances

  • Covert duplication, masked by change of authors, of language, or by adding extra data, causes problems. One danger is that patient data are analysed more than once in meta-analysis

  • 17% of systematically searched randomised trials of ondansetron as a postoperative antiemetic were covert duplicates and resulted in 28% of patient data being duplicated. None of these reports cross references the original source. Duplication lead to an overestimation of ondansetron's antiemetic efficacy of 23%. Trials reporting greater treatment effect were significantly more likely to be duplicated

  • Covert duplication of data has major implications for the assessment of drug efficacy and safety


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