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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 detection 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 such a way that most of the targets occur at one time interval (expected = 60%) and at other time intervals (unexpected=20%) for other trials, the rest 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 the expected time intervals may create an internal systematic bias for the observer the d prime are expected to be high at those target intervals and lower in the other intervals (may be due to tuning attention band to the frequently occurring or expected trials, attentional blink, attentional 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 instructions 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 the unexpected trials instead of lower d primes, I am getting negative d prime values.The most common explanation for negative d primes that I could trace in literature was due to sampling error, response confusion or malingering. In this experiment I do not suspect a 
response confusion or malingering. In terms of sampling error, I have read some literature in terms of assuming the confidence interval for d prime to be plus or minus twice the standard error and then if the SE error includes values equal to or close to zero then the negative d prime is actually zero or close to zero.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 calculate this ? 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 simply flipped to indicate the discriminability ? Can these negative d primes (pertaining to the current experiment) be interpreted in some logical way (eg: Inattention to unexpected trials) ?  

Imran Dhamani