BMJ : British medical journal
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Funnel plots (plots of effect estimates against sample size) may be useful to detect bias in meta-analyses that were later contradicted by large trials. We examined whether a simple test of asymmetry of funnel plots predicts discordance of results when meta-analyses are compared to large trials, and we assessed the prevalence of bias in published meta-analyses. ⋯ A simple analysis of funnel plots provides a useful test for the likely presence of bias in meta-analyses, but as the capacity to detect bias will be limited when meta-analyses are based on a limited number of small trials the results from such analyses should be treated with considerable caution.
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To quantify the impact of duplicate data on estimates of efficacy. ⋯ 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.