Methodology of superiority vs. equivalence trials and non-inferiority trials

The randomized clinical trial (RCT) is generally accepted as the best method of comparing effects of therapies. Most often the aim of an RCT is to show that a new therapy is superior to an established therapy or placebo, i.e. they are planned and performed as superiority trials. Sometimes the aim of an RCT is just to show that a new therapy is not superior but equivalent to or not inferior to an established therapy, i.e. they are planned and performed as equivalence trials or non-inferiority trials. Since the types of trials have different aims, they differ significantly in various methodological aspects. The awareness of the methodological differences is generally quite limited. This paper reviews the methodology of these types of trials with special reference to differences in respect to planning, performance, analysis and reporting of the trial. In this context the relevant basal statistical concepts are reviewed. Some of the important points are illustrated by examples.