There’s three different types of clinical trial designs: superiority, equivalence, and non-inferior. All evaluate how one treatment performs compared to another.
A superiority trial is designed to show that one treatment is better than another. This represents the majority of trials you’ll hear about.
An equivalence trial is designed to show that two treatments are equal in their performance. The equivalence margin, or the amount of difference that is acceptable to show equivalence, is represented for statistical purposes as “delta”.
A non-inferiority trial is designed to show that one treatment is not inferior to, or worse than, the control. The non-inferiority margin, or “delta”, is the margin that researchers use to state that the two treatments are non-inferior. Non-inferiority trials are sometimes preferred because fewer participants are needed which reduces costs, among other reasons.