Few trials discussed whether the observed differences were clinically important. Most trials with negative results did not have large enough sample sizes to detect a 25% or a 50% relative difference. Only 20 of the 102 reports made any statement related to the clinical significance of the observed differences.Ĭonclusions. Overall, only 32% of the trials with negative results reported sample size calculations, but the percentage doing so has improved over time from 0% in 1975 to 43% in 1990. These percentages did not consistently increase overtime. Of the simple two-group parallel design trials having negative results with dichotomous or continuous primary outcomes (n=70), only 16% and 36% had sufficient statistical power (80%) to detect a 25% or 50% relative difference, respectively. The number of published RCTs more than doubled from 1975 to 1990, with the proportion of trials with negative results remaining fairly stable.
Twenty-seven percent of the 383 RCTs (n=102) were classified as having negative results. We reviewed all 383 RCTs published in JAMA, Lancet, and the New England Journal of Medicine in 1975, 1980, 1985, and 1990. Power calculations were based on results from the primary outcomes reported in the trials.
STATISTICAL CALCULATIONS FOR TRIALS TRIAL
Criteria were developed both to classify trial results as positive or negative and to identify the primary outcomes.
Power to detect 25% and 50% relative differences was calculated for the subset of trials with negative results in which a simple two-group parallel design was used. To describe the pattern over time in the level of statistical power and the reporting of sample size calculations in published randomized controlled trials (RCTs) with negative results.ĭesign.