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

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 significant more likely to be duplicated. Inclusion of duplicated data in meta-analysis led to a 23% overestimation of ondansetron's antiemetic efficacy.