Inadequate statistical power of negative clinical trials in urological literature

Purpose: Negative studies provide valuable information. However, conducting studies with inadequate power is unethical and an inefficient use of resources. The purpose of this study was to determine the prevalence of negative studies with inadequate power in urological literature. Materials and Methods: The Journal of Urology (R), Urology (R) and BJU International (formerly British Journal of Urology) from 1982 to 2002 were searched using the Ovid MEDLINE (R) database. All clinical trials that contained the phrase "no difference" were identified. Data necessary for power calculation were extracted from applicable studies. Results: Of the 417 articles identified in the MEDLINE (R) search, 127 were negative studies that contained enough information to be analyzed. There were 70 (55%) articles from The Journal of Urology@, 35 (28%) from BJU International and 22 (17%) from Urology (R). Of the studies that used continuous variables 65% had adequate power (greater than 80%) to detect a 50% difference between groups and 32% had adequate power to detect a 25% difference. Of the studies that used dichotomous variables only 33% had adequate power to detect a 50% difference between groups and 23% had adequate power to detect a 25% difference. Levels of adequate power in negative studies did not improve over time (p = 0.258). Conclusions: Many negative studies in urological literature are inconclusive because they lack adequate power to detect even large differences between groups. Inadequately powered studies often result in false conclusions that alter clinical behavior and deter further research. Therefore, it is imperative to consider power when interpreting literature. When designing future investigations power calculations should be performed to ensure sufficient patient recruitment to attain clinically meaningful results.