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Re: identification task...negative d' values.
I grew up on Green's and Swets' signal detection book, as introduced
by the phenomenal teacher Jim Egan was. I remember an episode that
made me turn my back to negative d' presentations of any data. I had
just stumbled upon an interesting phenomenon: a systematic
underestimation of an unfilled time interval marked by two sine
bursts frequencies f1 and f2, when compared to an interval marked by
bursts of f1 only. While the results were robust when reached using
the method of adjustments, I felt as being on soft ground as long as
I did not have a discriminability measure as well. So, I ran a 2AFC
experiment (without feedback, to allow the subjects to respond to
what they heard rather than to what they were told to hear) and
obtained a d' psychometric function extending to values below zero,
as a function of time difference. When I presented those data at an
Acoustical Society meeting, Dave Green asked what I really meant by
negative d' , commenting that d' was a measure of differential
sensitivity which, whether positive or negative, marked
discriminability, and that a negative d' only meant that the subject
mislabeled the stimuli.
My interpretation of Daniel's results is similar: his subjects
identified the stimuli consistently incorrectly, meaning that they
could hear the difference between them but put them in the wrong bins.