What does 'non-inferior to' really mean? A clinician thinking out loud.

As clinicians, we are frequently faced with papers stating that something is 'non-inferior' to something else. By definition, a non-inferiority trial aims to demonstrate that the test product is not worse than the comparator by more than a small pre-specified amount. This amount is known as the non-inferiority margin, or delta. Clinicians must know who has chosen the margin, and why. Only when the advantages of the trial treatment clearly overcome the amount of 'worsening' which is implicit in the concept of non-inferiority and delta can we recommend this new 'non-inferior' (or, rather, 'just a little bit worse') treatment to our patients.