signal detection theory

Give an example for a decision situation that is suitable for applying SDT

A physician interpretws an X-ray

a crime witness tries to identify a criminal in a line-up

jury has to decide whether a suspect is guilty

according to SDT, what are two factors that affect human discrimination decisions?

Sensitivity: how good am I at identifying/discriminating?

Decision criterion/response criterion: How do I prefer to respond, if I am in doubt?

why is it important to consider both factors when analyzing decisions?

because the responses always depend on two factors

which two features should an experiment have so it can be analysed with SDT

repeated ‘presentation/ not-presentation’ stimulus

difficult decisions

in SDT what does ‘sensitivity’ mean?

strenght of the signal

ability of the observer

In SDT what does ‘decision criterion’ mean?

consequences of decisions

frequency of signal

Accordinf to SDT what are hit, false alarms, correct rejections and misses?

Hit- signal+ response

Miss-signal+ no response

False alarm- no signal+ response

Correct rejection- no signal+no response

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what are theoretical assumptions that explain differences in sensitivity?

In reality there is a signal or not, howeve, subjective experiences varies

Distributed around the means


1. the data follows Gaussian distribution
2. the variances of the two distributions are equal

What does it mean to say that someone has high or low sensitivity?

People for which it is easy to tell things apart have high sensitivity and people that have trouble discriminationg have low sensitivity

What does it mean to say that someone responds liberally, enutrally or conservatively?

Liberal: a lot of false alarms but hardly any or no misses

Neutral: equal yes and no responces

Conservative: many misses few false alarms

Whaich factors affect a person’s sensitivity?

Strength of the signal

ability of the observer

Which factors affect a person’s response criterion?

consequences of decisions

frequency of signal

What is the meaning of SDT values d’(d prime) and B(beta)

distance between standardized means of the N distribution and the S+N distribution

more liberal,neutral or conservartive

What does a low/high value of d’ mean?

Low=0 (person cannot discriminate at al)

High=4,66(almost perfect at discriminating)

What does a low/high value of B mean?

Low =.50(liberal yes responder)

high=1.99(conservative no responder)

which values do you need to compute a persons d’ value?

Yes responses (hits and false alarms)

d’=zHits-zFalseAlarms

what is the d’ value for someone who is guessing?

d’=0

Which values do you need to calculate someones B value?

Yes responses (Hits and False alarms) \n ß = yHits / yFalse alarms

 What is the ß value of a person who responds neutrally?

ß = 1

how can SDT be applied to memory experiments?

Is presented stimulus old?

Hit= yes/old False alarm=yes/new

Miss=no/old Correct rejection=no/new

how is the SDT term ‘present hits’ related to ‘sensitivity of diagnostic questionaires?

A correct Yes response to true yes cases

How is the SDT-term "false alarms" related to the "specificity of diagnostic questionnaires"?

Incorrect Yes response to true No cases

Why is "sensitivity" in SDT not the same as the "sensitivity of diagnostic questionnaires"?

SDT: sensitivity (d’) = zHits – zFalse alarm \n Diagnostics: sensitivity = %Hits