Questions about pd 2004-11-04 05:52:49 I'm trying to wrap my head around the value of pd, aka p2, aka the probability distinguisher, referenced in the literature (and ff123's posts) as being the probability that a listener is actually detecting a difference in an ABX test.My two questions:FIRST: What I find most troubling about pd is that it is explicitly a context-dependent quantity - at the very least, potentially varying considerably between individuals - but more importantly, varying drastically between samples. For transparent encoders, most samples are going to be almost completely indistinguishable, implying a pd between 0 and 0.1, and a few problem samples are very distinguishable (close to 1). In general, every time the context of the listening environment changes in the course of the test, theta is rendered invalid.How does the notion of a proportion distinguisher handle that? One potential solution is to do what John Corbett outlined in the ff123-vs-Arny usenet thread, and treat problem samples as members of a "population" with varying pd values for each. The idea that half a population can hear something and the other half can't, implying split values of pd which implies a more complicated hypothesis test scheme, seems very analogous to a certain proportion of test samples being problem samples with high values.For instance: say that for most samples, the transparency holds and you'll only notice a difference maybe 10% of the time, but for problem samples, which might be 1% of the theoretical "set" of samples, the difference is glaringly obvious and pd is more like 0.9 or so. That results in a pmax of... drumroll, please... 0.55. That's really, really low, but it does make a good bit of sense. The question is, is it correct?SECOND: How exactly are you supposed to come up with pd-values (or theta-values) anyway? Is there a rhyme or reason to it, or do I just guess?