Re: Back to the piano ... ("Timothy C. Justus" )

Subject: Re: Back to the piano ...
From:    "Timothy C. Justus"  <Timothy.C.Justus(at)DARTMOUTH.EDU>
Date:    Fri, 23 Oct 1998 05:12:11 EDT

The essential parts of Jim Beauchamp's message are quoted at the = end of my message... To answer the question about my posting, yes, different points on = the same curve in an ROC could represent different subjects with = different biases, or perhaps the same subject under different = experimental manipulations of bias. For example of the latter (a = rather rough one), providing a reward for a correct identification = of a piano tone (hit), while being indifferent to an incorrect = identification of a synthetic tone (false alarm), would bias the = subject to answer "piano tone" more often, increasing both H and F.=20 I agree that the choice of D =3D 50% as the "indistinguishability = threshold" is not logical; this number does not indicate chance = performance if we are using D =3D H - F. With this measure, scores = near *zero* would indicate low discrimination, while scores = significantly above zero (or below for that matter) would indicate = higher discrimination. Something as simple as a t-test comparing = the population of D values to 0 could tell you if these synthesized = tones are passing for the real thing. In the case of P =3D .5 + .5D, Jim is right that a P of .50 would = indicate guessing, since as we have said, D would be 0 in this = case. 100% would indicate perfect discrimination and = categorization. Incidentally, 0 would also indicate perfect = discrimination. The subject can tell the difference but merely has = the labels backwards. Timothy Justus Dartmouth College --- "James W. Beauchamp" wrote: What threw me off was my friend's assertion that D =3D 50% could be = taken as the "indistinguishability threshold", and since the results were = below that threshold, the synthetic tones were therefore considered to be "indistinguishable" from the originals. However, it seems to me = that D =3D 50% is rather arbitrary, and my friend's scores do in fact indicate = that some degree of discrimination is going on. Would you agree? Anyway, doesn't presenting the D scores (as I defined it above) = accomplish the same thing? If a point is above the diagonal, then D =3D H - F = > 0. The advantage of the D scores is that they are probably easy to = understand by the typical reader of a sound synthesis article. Another way to present the data would be percent correct, P. In = this case, P =3D .5*(H + (1-F)) =3D .5 + .5*D So we see that this is just a rescaling of the D data where, in = this case, 100% corresponds to perfect discrimination (or catagorization) and = 50% is the guessing threshold. --- end of quote --- McGill is running a new version of LISTSERV (1.8d on Windows NT). Information is available on the WEB at

This message came from the mail archive
maintained by:
DAn Ellis <>
Electrical Engineering Dept., Columbia University