IMPACT OF COVERT DUPLICATE PUBLICATION ON METAANALYSIS - A CASE-STUDY

Objective: To quantify the impact of duplicate data on estimates of ef ficacy. Design: Systematic search for published full reports of random ised controlled trials investigating ondansetron's effect on postopera tive 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 tr ials and patient data. Estimation of antiemetic efficacy (prevention o f emesis) of the most duplicated ondansetron regimen. Comparison betwe en the efficacy of non-duplicated and duplicated data. Results: Data f rom 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 investigat ed in 16 reports not subject to duplicate publication three reports su bject to duplicate publication, and six duplicates of those three repo rts. The number needed to treat to prevent vomiting within 24 hours wa s 9.5 (95% confidence interval 6.9 to 15) in the 16 non-duplicated rep orts 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 w as 6.4 (5.3 to 7.9). When all original and duplicate reports were comb ined (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 pub lished full reports of randomised trials and 28% of the patient data w ere duplicated. Trials reporting greater treatment effect were signifi cant more likely to be duplicated. Inclusion of duplicated data in met a-analysis led to a 23% overestimation of ondansetron's antiemetic eff icacy.