Subject: Re: Interpreting a negative d' From: Pierre Divenyi <pdivenyi@xxxxxxxx> Date: Thu, 12 Mar 2009 14:09:55 -0700 List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>I remember that some 30-odd years ago I presented data at an Acoustical Society meeting in which I showed upside-down responses in a 2IFC task in form of negative d'. And I have never forgotten David Green's (very kindly expressed) comment: d' always measures discriminability which is a distance and which, by definition, can never be negative. Therefore, every instance of a d' < 0 can only be assigned to the observer's bias. So the question is not how to interpret a negative d' but how to interpret the bias. -Pierre At 11:04 AM 3/12/2009, Landsberger, David wrote: >I have conducted an experiment where I have obtained for one subject in one >condition a negative d' which I cannot explain. I was hoping that someone >here might be able to offer me some insight. > >The experiment is a 3 interval forced choice task where a sound is presented >in each of the three intervals. In two of the intervals, the sounds are >identical. In the third interval, the sound is different. (Obviously the >order of stimuli is randomized.) The patient's task is simply to tell me >which of the three sounds is different. > >In a 2IFC task, a negative d' might indicate that the subject has >misunderstood the task. For example, if two sounds were played and the >subject were asked to pick the sound that was higher pitched, a negative d' >would mean that the subject reliably picked the lower pitched sound as >having a higher pitched. > >However, for a negative d' in a 3IFC task where two stimuli are the same, >the subject would have to reliably not pick the different one as being >different. What would that suggest about their perception? I don't think >its a misunderstanding of the task as the same subject was able to perform >fine in the same task using a different set of stimuli. > >Any advice would be greatly appreciated. > >Thanks, >David