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Heuristics and bias
Dr Carl Thompson
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Before we start
A quickexercise
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Poor judgements in conditions of uncertainty - how
and why?
How (bias) primacy and recency
ignoring base rates
overconfidence Framing
etcetc
why (heuristics) Representativeness
Availablity
Anchoring and adjustment
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What are heuristics?
Limited number of principles thatindividuals use to make sense ofcomplexity
Generally useful but lead to severe andsystematic errors
Subjective probability estimates similar to
physical quantities (size or distance) Clarity!
Kahneman and Tversky
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Representativeness
P obj A belongs to class B (Dx)?
P event A originates from process B(causality)
P process B will generate event A(treatment)
People rely on representativeness or thedegree to which A resembles B.
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Representativeness (2)
Common problems with representativeness: engineers and lawyers*
Insensitivity to prior probabilities of outcomes
Large hospital small hospital, childrens IQ Insensitivity to sample size and law of small numbers
H-T-H-T-T-H H-H-H-T-T-T H-H-H-H-T-H Misconceptions of chance
Flight training+
Regression towards the mean Measuring Depression in oncology vs stroke patients
Base rate neglect
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What you can do
Dont be misled by highly detailedscenarios
Whenever possible, pay attention to baserates
Remember that chance is not selfcorrecting
Dont misinterpret regression towards themean
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availability
P (event) recalled by the ease with whichinstances can be brought to mind. Cardiac arrests, predictions of healing careers
Good news - availability is useful becauseinstances of large classes are usually reachedbetter and faster than instances of less frequent
classes
Bad news availability is affected by factorsother than frequency and P.
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availability
Plane crashes vs car crashes
filling in the gaps
Think of a number between 1 and 20 Biases due to retrievability of instances
Paths biases of imagine ability
10 questions Overconfidence makes biases from availability
worse
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Which has the most paths?
x x x x x x x x
x x x x x x x x
x x x x x x x x
x xx x
x xx xx x
x xx x
x x
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Subjects memory of a film clip of
A car accident (Loftus & Palmer, 1974)How fast were the cars going when
they
Smashed? Mean speed 40.8 mph
Collided? Mean speed 39.3 mph
Bumped? Mean speed 38.1 mphHit? Mean speed 34.0 mph
Contacted? Mean speed 31.8 mph
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Was there broken glass?
Response Smashed Hit control
Yes 16 7 6
No 34 43 44
p.s. there was no broken glass at all in the videoclip
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What you can do
Maintain accurate records and use them
Beware of wishful thinking
Break compound events into simple
events
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Anchoring and adjustment
Estimate the product 8 x 7 x 6 x 5 x 4 x 3x 2 x 1 ???
How thick is a piece of paper if folded inon itself 100 times?
Clinical anchors?
Initial estimate of pre-test likelihood ofdisease (including prevalence).
Cognitively cautious (hammond 1967)
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conclusions
Judgement and decision research isconducted by human beings who are proneto many of the same biases and errors astheir experimental subjects. (Plous 1993)
Heuristics exist for a reason and simplybeing aware of them can be enough
Biases CAN be overcome (ish) Re calibration
Alternative formulations of causes
Questioning what if?