Re: Interpreting Negative d prime values in simple Yes/No detection task (Brian Gygi )


Subject: Re: Interpreting Negative d prime values in simple Yes/No detection task
From:    Brian Gygi  <bgygi@xxxxxxxx>
Date:    Fri, 12 Aug 2011 18:21:58 +0000
List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>

----=_vm_0011_W7319225495_24954_1313173318 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Dear Imran, In my experience a negative d' usually means the subjects are not underst= anding the task properly. This means the stimuli are discriminable, but s= ubjects responding to the wrong thing. Looking through your procedures, I= can't say for sure if that is what happening, but you might consider it.= Brian Gygi, Ph.D. Speech and Hearing Research Veterans Affairs Northern California Health Care System 150 Muir Road Martinez, CA 94553 (925) 372-2000 x5653 -----Original Message----- From: Imran Dhamani [mailto:imrandhamani@xxxxxxxx Sent: Friday, August 12, 2011 02:09 AM To: AUDITORY@xxxxxxxx Subject: Interpreting Negative d prime values in simple Yes/No detection = task Dear List,In one of the experiments that I performed using the yes/no det= ection task (button press), I have got some negative d prime values which= I am not sure how to interpret.The experiment is actually created in suc= h a way that most of the targets occur at one time interval (expected =3D= 60%) and at other time intervals (unexpected=3D20%) for other trials, th= e rest 20% are the catch trials in which the target does not appear in an= y of the time intervals. Since the frequent presentations of the targets = in the expected time intervals may create an internal systematic bias for= the observer the d prime are expected to be high at those target interva= ls and lower in the other intervals (may be due to tuning attention band = to the frequently occurring or expected trials, attentional blink, attent= ional lapses or purposefully not heeding to the targets at the unexpected= time intervals due to the lower frequency of their occurrence).The last = possibility of lower d primes due to not heeding seems to be very low in = this experiment since the participants understood the task well and the i= nstructions given to them were neutral (i.e. press the button as soon as = the target is heard and not to press the button on catch trials) and they= were aware that the target can occur at any time interval. At some of th= e unexpected trials instead of lower d primes, I am getting negative d pr= ime values.The most common explanation for negative d primes that I could= trace in literature was due to sampling error, response confusion or mal= ingering. In this experiment I do not suspect a response confusion or mal= ingering. In terms of sampling error, I have read some literature in term= s of assuming the confidence interval for d prime to be plus or minus twi= ce the standard error and then if the SE error includes values equal to o= r close to zero then the negative d prime is actually zero or close to ze= ro.But I am not sure how this calculation exactly works and what is this = sampling error that we are calculating in the context of signal detection= theory. Can someone please throw some light on what sampling error means= in the context of a signal detection theory and also a simple way to cal= culate this ? Do negative prime values have no significance in terms of t= heir sign and thus if the negative d prime values are not too large they = can be simply flipped to indicate the discriminability ? Can these negati= ve d primes (pertaining to the current experiment) be interpreted in some= logical way (eg: Inattention to unexpected trials) ? Regards,Imran Dhama= ni ----=_vm_0011_W7319225495_24954_1313173318 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable <html><br>Dear Imran,<br><br>In my experience a negative d' usually means= the subjects are not understanding the task properly.&nbsp; This means t= he stimuli are discriminable, but subjects responding to the wrong thing.= &nbsp; Looking through your procedures, I can't say for sure if that is w= hat happening, but you might consider it.<br><div><font face=3D"Verdana" = size=3D"2">&nbsp;</font></div> Brian Gygi, Ph.D. <br> Speech and Hearing Research <br> Veterans Affairs Northern California Health Care System <br> 150 Muir Road <br> Martinez, CA 94553 <br> (925) 372-2000 x5653<div><font color=3D"#0000ff" face=3D"Verdana" size=3D= "2"></font>&nbsp;</div> <blockquote style=3D"border-left: 2px solid rgb(0, 0, 255); padding-left:= 5px; margin-left: 5px; margin-right: 0px;"><font face=3D"Tahoma" size=3D= "2">-----Original Message-----<br><b>From:</b> Imran Dhamani [mailto:imra= ndhamani@xxxxxxxx<br><b>Sent:</b> Friday, August 12, 2011 02:09 AM<br= ><b>To:</b> AUDITORY@xxxxxxxx<br><b>Subject:</b> Interpreting Nega= tive d prime values in simple Yes/No detection task<br><br></font>Dear Li= st, In one of the experiments that I performed using the yes/no detection tas= k (button press), I have got some negative d prime values which I am&nbsp= ; not sure how to interpret.The experiment is actually created in such a = way that most of the targets occur at one time interval (expected =3D 60%= ) and at other time intervals (unexpected=3D20%) for other trials, the re= st 20% are the catch trials in which the target does not appear in any of= the time intervals. Since the frequent presentations of the targets in t= he expected time intervals may create an internal systematic bias for the= observer the d prime are expected to be high at those target intervals a= nd lower in the other intervals (may be due to tuning attention band to t= he frequently occurring or expected trials, attentional blink, attentiona= l lapses or purposefully not heeding to the targets at the unexpected tim= e intervals due to the lower frequency of their occurrence).The last poss= ibility of lower d primes due to not heeding seems to be very low in this experiment since the par= ticipants understood the task well and the instructions given to them wer= e neutral (i.e. press the button as soon as the target is heard and not t= o press the button on catch trials) and they were aware that the target c= an occur at any time interval. At some of the unexpected trials instead of lower d pr= imes, I am getting negative d prime values.The most common explanation fo= r negative d primes that I could trace in literature was due to sampling = error, response confusion or malingering. In this experiment I do not sus= pect a response confusion or malingering. In terms of sampling error, I have rea= d some literature in terms of assuming the confidence interval for d prim= e to be plus or minus twice the standard error and then if the SE error i= ncludes values equal to or close to zero then the negative d prime is act= ually zero or close to zero.But I am not sure how this calculation exactl= y works and what is this sampling error that we are calculating in the co= ntext of signal detection theory. Can someone please throw some light on what sampling error means in the c= ontext of a signal detection theory and also a simple way to calculate th= is ? Do negative prime values have no significance in terms of their sign= and thus if the negative d prime values are not too large they can be si= mply flipped to indicate the discriminability ? Can these negative d prim= es (pertaining to the current experiment) be interpreted in some logical = way (eg: Inattention to unexpected trials) ?&nbsp; Regards, Imran Dhamani </blockquote></html> ----=_vm_0011_W7319225495_24954_1313173318--


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