1. Heuristics and bias Dr Carl Thompson

2. Before we start… A quick exercise

3. Poor judgements in conditions of uncertainty - how and why? How (bias) How (bias) primacy and recency primacy and recency ignoring base rates ignoring base rates overconfidence overconfidence Framing Framing … etc…etc … etc…etc why (heuristics) why (heuristics) Representativeness Representativeness Availablity Availablity Anchoring and adjustment Anchoring and adjustment

4. What are heuristics? Limited number of principles that individuals use to make sense of complexity Limited number of principles that individuals use to make sense of complexity Generally useful but lead to severe and systematic errors Generally useful but lead to severe and systematic errors Subjective probability estimates similar to physical quantities (size or distance) Subjective probability estimates similar to physical quantities (size or distance) Clarity! Clarity! Kahneman and Tversky Kahneman and Tversky

5. Representativeness P obj A belongs to class B (Dx)? P obj A belongs to class B (Dx)? P event A originates from process B (causality) P event A originates from process B (causality) P process B will generate event A (treatment) P process B will generate event A (treatment) People rely on representativeness or ‘the degree to which A resembles B’. People rely on representativeness or ‘the degree to which A resembles B’.

6. Representativeness (2) Common problems with representativeness: Common problems with representativeness: engineers and lawyers* engineers and lawyers* Insensitivity to prior probabilities of outcomes Insensitivity to prior probabilities of outcomes Large hospital small hospital, childrens IQ† Large hospital small hospital, childrens IQ† Insensitivity to sample size and law of small numbers Insensitivity to sample size and law of small numbers H-T-H-T-T-HH-H-H-T-T-TH-H-H-H-T-H H-T-H-T-T-HH-H-H-T-T-TH-H-H-H-T-H Misconceptions of chance Misconceptions of chance Flight training + Flight training + Regression towards the mean Regression towards the mean Measuring Depression in oncology vs stroke patients Measuring Depression in oncology vs stroke patients Base rate neglect Base rate neglect

7. What you can do Don’t be misled by highly detailed scenarios Don’t be misled by highly detailed scenarios Whenever possible, pay attention to base rates Whenever possible, pay attention to base rates Remember that chance is not self correcting Remember that chance is not self correcting Don’t misinterpret regression towards the mean Don’t misinterpret regression towards the mean

8. availability P (event) recalled by the ease with which instances can be brought to mind. P (event) recalled by the ease with which instances can be brought to mind. Cardiac arrests, predictions of healing ‘careers’ Cardiac arrests, predictions of healing ‘careers’ Good news - availability is useful because instances of large classes are usually reached better and faster than instances of less frequent classes Good news - availability is useful because instances of large classes are usually reached better and faster than instances of less frequent classes Bad news – availability is affected by factors other than frequency and P. Bad news – availability is affected by factors other than frequency and P.

9. availability Plane crashes vs car crashes Plane crashes vs car crashes ‘filling in the gaps’ ‘filling in the gaps’ Think of a number between 1 and 20 Think of a number between 1 and 20 Biases due to retrievability of instances Biases due to retrievability of instances Paths Paths biases of imagine ability biases of imagine ability 10 questions 10 questions Overconfidence makes biases from availability worse Overconfidence makes biases from availability worse

10. Which has the most paths? x x x x x

11. 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 mph “Hit”? Mean speed 34.0 mph “Contacted”? Mean speed 31.8 mph

12. Was there broken glass? Response“Smashed”“Hit”control Yes1676 No344344 p.s. there was no broken glass at all in the video clip

13. What you can do Maintain accurate records and use them Maintain accurate records and use them Beware of wishful thinking Beware of wishful thinking Break compound events into simple events Break compound events into simple events

14. Anchoring and adjustment Estimate the product 8 x 7 x 6 x 5 x 4 x 3 x 2 x 1 ??? Estimate the product 8 x 7 x 6 x 5 x 4 x 3 x 2 x 1 ??? How thick is a piece of paper if folded in on itself 100 times? How thick is a piece of paper if folded in on itself 100 times? Clinical anchors? Clinical anchors? Initial estimate of pre-test likelihood of disease (including prevalence). Initial estimate of pre-test likelihood of disease (including prevalence). Cognitively cautious (hammond 1967) Cognitively cautious (hammond 1967)

15. conclusions Judgement and decision research is conducted by human beings who are prone to many of the same biases and errors as their experimental subjects. (Plous 1993) Judgement and decision research is conducted by human beings who are prone to many of the same biases and errors as their experimental subjects. (Plous 1993) Heuristics exist for a reason and simply being aware of them can be enough Heuristics exist for a reason and simply being aware of them can be enough Biases CAN be overcome (ish) Biases CAN be overcome (ish) Re calibration Re calibration Alternative formulations of causes Alternative formulations of causes Questioning – what if? Questioning – what if?