Non-inferiority and choice of scale (absolute vs relative)

1

I am trying to understand more about non-inferiority trials and one thing that I am struggling with (for binary outcomes) is when it might be more appropriate to choose either an absolute (i.e. risk difference) vs relative (RR/OR) for establishing the non-inferiority margin. Are there any good reasons for choosing one over the other? Intuitively (for me) an absolute difference feels easier to work with…

2

An absolute scale is harder to work with because every patient will have a different absolute risk reduction (ARR is a function of baseline risk).

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3

And yet the majority of NI studies uses ARR, right?

4

Doesn’t this also make use of RR/OR difficult since whether or not an RR/OR is “large” depends on baseline risk?

5

An OR changing over the range of baseline risk when RR is held constant or vice versa does not tell us much. The best way to answer this question is to take a couple of hundred trials, all with different baseline risk, and see if baseline risk is associated with the reported magnitude of effect.