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Interpreting Negative d prime values in simple Yes/No detection task

Dear All,
Thanks a lot for your responses to my question. They are all very helpful. I also wanted to add a few points that I may have missed to mention in my previous post about the experiment.The responses to the targets which are outside the expected response window are considered as misses and those which occur before the target stimuli (expected or unexpected) are considered as false alarms for each of those observation intervals. In this way, I do have a way to separate the false alarms,hits and misses for each of those observation intervals.The main problem was in terms of distributing the false alarms that occur on catch trials(in which there is no target) on each of those observation intervals.Thus there are mainly two types of false alarms in the experiment.One is the button presses before the target for each of the temporal positions of the target and the other type is the one where there is a button press on catch trials.I do understand the limitation
 of the experiment in terms of having just one button for both targets and catch trials which was to simplify the cognitive demands of the task significantly.To put it in simple terms, when the participant makes a false alarm on a catch trials I do not know which temporal position he thought he heard the target in.One possibility was to assume equal rates of these false alarms across all temporal positions, but this assumption seems bad in the context of this experiment because as previously mentioned the target is most frequently presented at one temporal position than others.Thus the participant is more likely to false alarm on a catch trial on that expected temporal position and lesser false alarms on other unexpected positions.Thus a better choice in this case was to distribute the catch trial false alarms among the five temporal positions of the target occurrence in proportion to the chance of getting a hit at each of the temporal positions of the
 target.Is this assumption the best way to go or are there any other better ways to deal with this problem? With these points in addition to the previous experimental design that i had mentioned in my previous post,can the negative d prime values at the unexpected temporal positions of targets be explained in any logical way?

Imran Dhamani