I always have trouble making sense of noninferiority studies. If treatment B is noninferior to A, I can understand that. But when it says B is NOT noninferior to A, why can’t they just say B is inferior to A? What’s the difference?
Example of such a trial: https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(22)00537-2/fulltext
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This is a great discussion:
http://www.nephjc.com/news/2019/7/8/understanding-the-vortex-of-non-inferiority-trials
Choice quote: “Inability to prove non-inferiority does not conclude that the test intervention is inferior, it only means that it is “not non-inferior”. This outcome could be a result of an underpowered trial.”
So, statistical power is key. If you don’t get enough participants to reduce the influence of random noise, you can’t trust your results. If both study arms only had 3 patients complete the trial, then who cares if you didn’t show noninferiority? Besides “showed inferiority” and “showed noninferiority,” the third possibility is “didn’t show a damn thing.”
This is relevant to noninferiority trials, which don’t need as many participants because you’re only trying to show “eh, the new thing isn’t all that bad,” which takes fewer people to prove than “the new thing is this many points better than the old thing.”
The other key thing is that the margin of noninferiority is usually arbitrary/a judgment call. A study can fail to show noninferiority to the margin they used, but the new treatment can still look good enough to be worth talking about. One great example:
https://www.nejm.org/doi/full/10.1056/NEJMoa1502599#:~:text=For%20the%20treatment%20of%20chlamydia%20infection%2C%20the%20Centers%20for%20Disease,twice%20daily%20for%207%20days.
It compares an antibiotic that cured about 97% of chlamydia with a single dose, versus one taken twice daily for seven days that cured 100%. It narrowly missed the preset margin for noninferiority, yet which would you choose for a patient who never remembers to take medicine?
TL;DR: Failing to show noninferiority doesn’t show inferiority, because it might have just not showed a damn thing.
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