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pThreshold equivalence QUEST staircase

Hi There,

I was hoping for a quick clarification on the QUEST pThresholds, and how they relate to simple staircases. In the manual, it mentions that a pThresh of .63 is equivalent to 1 up 1 down, and .82 equivalent to 3 up 1 down. Can someone help me understand why this is the case? I would have thought .50 and .75 would be the equivalents (respectively).

As I’m sure you can tell, I’m a bit new to stair-casing. I’ve read the original Pelli paper, but I’m getting hung up on a few things like this.

Thanks,

Martin