A non-inferiority trial is similar to an equivalence trial. The research question in a non-inferiority trial is whether the experimental therapy is not inferior to the active control (whereas the experimental therapy in an equivalence trial should not be inferior to, nor superior to, the active control). Thus, a non-inferiority trial is one-sided, whereas an equivalence trial is two-sided. (For non-inferiority, we want experimental therapy to be not inferior to the active control.)
Assume that the larger response is the better response. The one-sided zone of non-inferiority is defined by \(-\Psi\), i.e., the difference in population means between the experimental therapy and the active control, \(\mu_E - \mu_A\), should lie within \(\left(-\Psi, + ∞\right)\).
Many of the same issues that are critical for designing an equivalence trial also are critical for designing a non-inferiority trial, namely, appropriate selection of an active control and appropriate selection of the “zone of clinical non-inferiority” defined by \(\Psi\).
Hypertensive Example Section
Consider the previous example with the standard and experimental antihypertensive therapies.
The researchers may decide that the experimental drug is clinically not inferior to the standard drug if its mean reduction in diastolic blood pressure is at least 3 mm Hg \(\left(\Psi = 2\right)\). Thus, the difference in population means between the experimental therapy and the active control therapy, \(\mu_E - \mu_A\), should lie within \(\left(-\Psi, + ∞\right)\). It does not matter if the experimental drug is much better than active control drug, provided that it is not inferior to the active control drug.
Because a non-inferiority trial design allows for the possibility that the experimental therapy is superior to the active control therapy, the non-inferiority design is preferred over the equivalence design. The equivalence design is useful when evaluating generic drugs.