# Sensitivity versus response bias: Psychophysical techniques.

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Sensitivity versus response bias: Psychophysical techniques

The experimental context The kinds of questions you may want to address: The kinds of questions you may want to address: –tone 1 versus tone 2 (or faces, or line length etc…) –tone 1 versus noise (or face versus standard face etc…) –new words versus old words (or faces etc…) You may face a situation where the stimuli are insufficiently discriminable for response latency to be a reasonable independent variable You may face a situation where the stimuli are insufficiently discriminable for response latency to be a reasonable independent variable In any of these cases Psychophysical techniques are a reasonable method to use. In any of these cases Psychophysical techniques are a reasonable method to use.

The technique Method: (the simplest) Method: (the simplest) –one interval –one of two stimulus alternatives are presented –participant identifies stimulus alternative –response accuracy is independent variable Decision axis (z units) Stimulus representation Stimulus 2 Stimulus 1

The technique… We consider the response matrix from the point of view of one response type We consider the response matrix from the point of view of one response type We use the number of Hits (H) and False Alarms (F) We use the number of Hits (H) and False Alarms (F) Sensitivity d= z(H)-z(F) Sensitivity d= z(H)-z(F) Response bias c = -0.5[z(H)+z(F)] Response bias c = -0.5[z(H)+z(F)] Decision axis (z units) Response Stimulus12 1HitsMisses 2 False Alarm Correct rejection

Importance of response bias If you concentrate on response accuracy without taking into account response bias, you may get a misleading idea If you concentrate on response accuracy without taking into account response bias, you may get a misleading idea Important in cases where response bias is crucial – for example histological examinations for precancerous cells, x- ray examinations etc… Important in cases where response bias is crucial – for example histological examinations for precancerous cells, x- ray examinations etc… Decision axis (z units) HFdc 0.800.701.35-0.17 0.750.751.350.00 0.700.801.350.15 0.650.831.340.28 0.600.871.360.42

References The classic text: The classic text: –Green, D.M., and Swets, J.A., (1966). Signal Detection Theory. Wiley: London A user friendly text: A user friendly text: –Macmillan, N.A., and Creelman, C.D., (1991). Detection Theory: A users guide. Cambridge University Press. Know of anything more up to date, please let me know… Know of anything more up to date, please let me know…

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