But then how do you account for the possible differences in the quality of the interface? Keeping the interfaces relatively similar except for the buttons and i

But then how do you account for the possible differences in the quality of the interface? Keeping the interfaces relatively similar except for the buttons and inputs makes it easier to compare the two. This plane metaphor is ridiculous and unhelpful. We're not trying to measure "how useful wings are", we're talking about what the buttons on the inside of the plane should look like, we need two cockpits with different buttons.