Utilities of the P-value Distribution Associated with Effect Size in Clinical Trials<FNR HREF="fn1"></FNR><FN ID="fn1">The views expressed in this article do not represent those of the U.S. Food and Drug Administration.</FN>

The P-value, which is widely used for assessing statistical evidence in randomized comparative clinical trials, is a function of the observed effect size of the experimental treatment relative to the control treatment. The relationship of the P-value with the observed effect size at study completion and the effect size anticipated at the design stage has potential usefulness in providing guidance for planning and interpretation of a clinical trial. The post-trial power associated with a statistically significant P-value from a completed study is also a random variable and its use may assist in planning a follow-up trial to confirm the statistically significant findings in an initial study. A measure of robustness is explored to quantify the degree of sensitivity of the observed P-value to potential bias that may be contained in the observed effect size.