We consider the problem of stopping a clinical trial before its scheduled termination due to the apparent ineffectiveness of the experimental therapy, as compared with a control. We propose a simple-to-implement, intuitive decision rule based on the unadjusted attained significance levels from any appropriate statistical test. The proposed procedure may be used at any time during the study as an aid to help determine whether the study of an experimental treatment should be terminated early with the conclusion of treatment ineffectiveness. Much of the power of the usual fixed-sample test is retained while maintaining the nominal test size.