heuristics and bias

7/29/2019 Heuristics and Bias

1/15

Heuristics and bias

Dr Carl Thompson


2/15

Before we start

A quickexercise


3/15

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


4/15

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


5/15

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.


6/15

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


7/15

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


8/15

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.


9/15

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


10/15

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


11/15

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


12/15

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


13/15

What you can do

Maintain accurate records and use them

Beware of wishful thinking

Break compound events into simple

events


14/15

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)


15/15

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?

heuristics and bias

Documents