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Calculating sensitivity for a single interval 5AFC syllable discrimination task

Dear List,
              I have currently collected some data using a single interval 5 alternative forced choice syllable discrimination task. The task is to detect a target syllable amongst other syllables. The target can occur as any of the 5 alternatives, but occurs more frequently (60% of the time) as the first alternative and less frequently as others. Thus there is an induced response bias that we have generated in the task using unequal distribution of trials at each of the the alternatives. The problem that i am facing is how to analyze this data using sensitivity as the index of discriminability. I understand that the traditional signal detection theory cannot handle induced response bias such as in the current task as it assumes equal distribution/probability of target occurrence at each of the 5 alternatives. Moreover as I understand d primes itself is relatively independent of response bias in the signal detection theory which is counterproductive in the current task as there is a specific reason of inducing this bias in our task. The other option I had was calculating d primes using the High threshold theory by correcting the hit rates for guesses/false alarms but that too is difficult since the guess rate for a multiple alternative forced choice is usually calculated as guess/lambda=1/number of alternatives but in the current task we also have additional catch trials (20%) in which the target syllable does not occur at all which allows us to calculate individual false alarm and hit rates for the target occurrence at each of the 5 alternatives within an interval by using the participants reaction times (using the method described by Watson and Nichols (1976) for signals presented without defined observation intervals). So in a way it is a combination of a mAFC and Yes/No task, thus making it complex for me to analyze. One other way I explored was using the correction for z scores for response bias instead of correcting the hit rates as described by Klein (2001) but the method described in the research article does not allow me to calculate 5 separate d primes for the target occurrence at each of the alternatives. One point to note in addition in this task is that since the probability of target occurring as the first alternative is much higher than others, along with higher hit rates for target presented at that alternative there is also a higher tendency of participants to false alarm on the catch trials on that alternative as well (i.e unequal distribution of false alarms).

Can someone please suggest me a way of analyzing this data ?

Imran Dhamani

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