What is a Non-Inferiority Margin?
A non-inferiority margin is such a difference m that we would be happy implementing a tested treatment if we are to be able to rule out outcomes below -m (negative and smaller in magnitude than m) with our desired significance threshold.
The non-inferiority margin should not be chosen lightly: it should be informed by as objective criteria as possible and it should be communicated to all stakeholders before the A/B test begins.