Statistical power of negative randomized controlled trials presented at American Society for Clinical Oncology annual meetings

Purpose To investigate the prevalence of underpowered randomized controlled trials (RCTs) presented at American Society of Clinical Oncology (ASCO) annual meetings. Methods We surveyed all two- arm phase III RCTs presented at ASCO annual meetings from 1995 to 2003 for which negative results were obtained. Post hoc calculations were performed using a power of 80% and an alpha level of .05 (two sided) to determine sample sizes required to detect small, medium, and large effect sizes. For studies reporting a proportion or time-to-event as primary end point, effect size was expressed as an odds ratio (OR) or hazard ratio (HR), respectively, with a small effect size defined as OR/ HR >= 1.3, medium effect size defined as OR/ HR >= 1.5, and large effect size defined as OR/ HR >= 2.0. Logistic regression was used to identify factors associated with lack of statistical power. Results Of 423 negative RCTs for which post hoc sample size calculations could be performed, 45 (10.6%), 138 ( 32.6%), and 233 (55.1%) had adequate sample size to detect small, medium, and large effect sizes, respectively. Only 35 negative RCTs (7.1%) reported a reason for inadequate sample size. In a multivariable model, studies that were presented at oral sessions (P <.0038), multicenter studies supported by a cooperative group (P <.0001), and studies with time to event as primary outcome (P <.0001) were more likely to have adequate sample size. Conclusion More than half of negative RCTs presented at ASCO annual meetings do not have an adequate sample to detect a medium- size treatment effect.