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Re: Interpreting a negative d'

If you are convinced that your subject understood the task correctly,
I would try to rule out these possibilities:

- Was the negative d' simply a matter of insufficient sampling?
(i.e., you randomly got an answer that looked significant)  It would
take more trials to determine if this is a likely explanation.

- Was there, in fact, some difference between the 2 "same" signals
that the listener picked up on, and you were not aware of?  Depending
on your test setup, this could be an artifact of the system, and it
may have only been audible for that stimulus (and/or for that


On Thu, Mar 12, 2009 at 2:04 PM, Landsberger, David
<DLandsberger@xxxxxxx> 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