If this makes sense, how is the study able to give a reasonable measure of how long an issue/task should have taken, vs how long it took with AI to determine that using AI was slower?

Or it's comparing how long the dev thought it should take with AI vs how long it actually took, which now includes the dev's guess of how AI impacts their productivity?

When it's hard to estimate how difficult an issue should be to complete, how does the study account for this? What percent speed up or slow down would be noise due to estimates being difficult?

I do appreciate that this stuff is very hard to measure.