An Overview of Signal Detection Theory

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An Overview ofSignal Detection Theory

• Psy. 159
• Josh Wortman

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Background

• Why use SDT?
• Psychophysics
• attempts to quantify sensation
• Sensation
• detecting stimuli in the environment.
• Perception
• interpreting these detected stimuli.

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Gathering Threshold Data

• Method of Limits
• Method of Adjustment
• Constant Stimuli
• Method of Forced Choice

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Step Function
100
50
Percentage Detected ()
0
Weak
Strong
Intensity
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Ogive
100
50
Percentage Detected ()
0
Weak
Strong
Intensity
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Key Terms in Detection

• Noise
• environment, physiological, or cognitive
• Discrimination
• prothetic, metathetic
• Threshold
• min, max, j.n.d.

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• Decision Criteria
• False-alarm
• Psychometric Function

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There is noise inthe environment and in the
observer
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Taking the average noise level over time produces
the NORMAL CURVE!
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Observer Response  Observer Response 
State of the world  Yes, there was a signal  No, there wasn't a signal 
Signal Present  Hit 1-b Miss b
Signal Absent  False Alarm a Correct Reject.  1-a
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The ROC curve
Receiver Operating Characteristic
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Payoff Matrix

• Affects the Criterion Level
• Rewarding
• lowers criterion level
• Penalizing
• Raises criterion level

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Observer Response  Observer Response 
State of the world  Yes, there was a signal  No, there wasn't a signal 
Signal Present  Hit 1-b Miss b
Signal Absent  False Alarm a Correct Reject.  1-a
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Practical Applications

• Simple stimulus detection
• tone recognition
• touch sensitivity
• Cognitive Processes
• evidence assessment
• memory
• social behavior

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• Witness statement photos ...
• police reports - bias - ... - emotionality
  
• DECISION
• or more generally
• d1 d2 d3 d4 ... dn Decision
• where each d is a dimension of criteria. There
  may be an unlimited amount of ds.

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Effect of Duration of Exposure (d) and Length of
Retention Interval on Mean Accuracy ( Correct)
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Please see

• http//www.psychwatch.com/
• tutorials_page.htm

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Thank you
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One last note

• This presentation is dedicated to
• Joana Costa
• who helped me to stay awake while preparing