HUMAN FACTORS ENGINEERING – IEE437/IEE547
Reading Guide SDT
Signal Detection Theory
Readings: Textbook pp. 83-86 and 102-109
Atomic Chef by Steven Casey, pp. 41-53 (this will be on the quiz)
1. Signal Detection Theory (SDT) is explained in our textbook on pages 83-86 and 102-103. Please study
those pages carefully.
2. SDT is applicable in any situation in which there are two discrete states of the world. The signal is
ambiguous so the observer has to make a decision based on criteria. The observer says YES (I detect
a signal) or NO (I do not). Sometimes the signal is present in noise.
3. Some examples of applications are:
 Detection of a concealed weapon by an airport security guard. Is that shape on the x-ray
machine a weapon?
 Did that basketball player foul?
 Should I hire this applicant for the job?
 A witness to a crime is asked to identify a suspect. Was this person present at the time of the
crime or not?
 A critical event in air traffic control. Is that bleep on the radar something to worry about?
4. Observations are made. The observer makes a decision based on experience, knowledge, or some
other criteria. Sometime the data include distracters. Example: “A doctor is examining a patient and
trying to make a diagnosis. The patient shows a set of symptoms, and the doctor tries to decide
whether a particular disorder is present or not. To complicate the problem, the symptoms are
ambiguous, some of them pointing in the direction of the disorder, others pointing away from it.
Moreover the patient is a bit confused and does not describe the symptoms clearly or consistently.
The correct diagnosis is not obvious.” (Wickens 2002)
5. Data are presented in a 2x2 matrix.
The four outcomes are Hit (signal is present and observer says "yes"), Miss (signal is present and
observer says "no"), False Alarm (signal is not present and observer says "yes"), and Correct
Rejection (signal is not present and observer says "no). Raw numbers are entered on the matrix and
then percentages are calculated.
States of the World
Responses
Signal
No signal
Hit
False
Alarm
Miss
Correct
Rejection
YES
NO
SDT
1
6. A normal curve will be produced depending on the subject's ability to discriminate between signal
and noise. In figure (a), the graph shows high sensitivity and (b) shows low sensitivity. It is possible
that if the subject never makes a mistake, the graph will show two normal distributions that do not
overlap at all.
7. Draw the third type of curves.
8. Correct responses are hits and correct rejections. The probability of a correct response is the
percent of hits plus the percent of CR, divided by 2.
P(correct response) = P(hit) + P(correct rejection)
2
SDT
2