Probability:

Question 1

What is probability?

Answer 1

• how likely something is.
• usually on a scale of 0-100% or 0-1.
• it follows that you need to act before you are certain.
• consider probability for better decision-making in the context of uncertainty.

Question 2

is it possible to rationally update one’s views to reach complete certainty (0-100%)?

Answer 2

• no but you can approach this.

Question 3

What were the results of the Morgan et.al 2021 paper?
(Accuracy of practitioner estimates of the probability of diagnosis before and after testing).

Answer 3

• respondents overestimated the probability of diagnosis before and after testing.
• this overestimation is consistent with cognitive biases, including base-rate neglect, anchoring bias and confirmation bias.

Question 4

what are the 2 implications of these results from morgan et.al?

Answer 4

1. overestimated probability used in deciding whether to initiate therapy can result in medication overuse and excessive procedures with their associated harms.
2. these errors would corrupt shared decision-making with the patients because practitioners need to understand the likelihood of various outcomes in order to communicate them.

Question 5

what is the problem with how decision-making is taught?

Answer 5

• decision making is often focused on communication skills and not on actually understanding the probability of disease.

Question 6

what was the worrying observation found by Attia et al (2004)?

Answer 6

there were a number of clinicians indicating pre-test probabilities of 100%.

Question 7

What does this finding reflect from Attia et al?

Answer 7

• This presumably reflects a cautious attitude, assuming that all patients have the disease until proven otherwise.

Question 8

When would indicating pre-test probabilities work?

Answer 8

• this would only work if the tests ordered have powerful negative likelihood ratios and if these tests indeed does give negative results.

Question 9

What does the overestimation of disease risk do?

Answer 9

• leaves clinicians unable to judge false positive test results, and may result in more intervention than necessary, indicating a lack of appreciation for how diagnostic tests influence the probability of disease.

Question 10

What are biases?

Answer 10

• Not a personal shortcoming but a human trait.
• when we substitute judgements of representativeness for judgements of actual probability.

Question 11

what is representative?

Answer 11

-the degree to which something is representative of, or similar to, the stereotype.

Question 12

Outline the steve bias example, is he more likely to be a farmer or a librarian?

Answer 12

• based on the description alone it sounds as if he is a librarian (judgment of representativeness).
• however, we have to take into account other factors such as when this was where this was etc. farmers had a bigger role in society during this time than librarians.
• is there a library in his city, does he read?

Question 13

Why is being aware of bias important?

Answer 13

• because an uncommon representation of a common disease is more likely than a common representation of a rare disease.
• if this weren’t so then diagnosis would be a matter of pattern matching.
• when you hear hoofbeats think of horses not zebras.

Question 14

how can the essential keys to disciplined bayesian reasoning be simply summarized?

Answer 14

• anchor your judgement of an outcome on a plausible base rate (pretest probability).
• question the diagnosticity of your evidence.

Question 15

What is a pretest probability?

Answer 15

• it is the best estimate of a disease probability before you do a test.
• it is a starting place from which to update probability.

Question 16

what do you have after a test?

Answer 16

• post test probability.

Question 17

how do we determine pretest probability?

Answer 17

• there are multiple ways to do this but you want to start with a good reference class.

Question 18

what is the best reference class?

Answer 18

• the set of patients that most closely matches this patient.

Question 19

What is a basic reference class?

Answer 19

• the prevalence of disease in a population.

Question 20

what is prevalence?

Answer 20

• the proportion of a population affected by a condition.

Question 21

what are the pros of using prevalence?

Answer 21

• relatively easy to search for.
• can specify sub-populations to get a more accurate estimate.

Question 22

what are the cons to using prevalence?

Answer 22

• maybe an underestimate if it is something that people frequently seek medical attention for.
• less helpful for acute medical conditions.

Question 23

What is a more specific reference class?

Answer 23

• studies that give an eventual diagnosis in patients presenting with complaints similar to your patients.

Question 24

what are the pros of using a study?

Answer 24

• takes the symptoms into account to provide a more accurate initial judgement.
• takes into account that people tens to seek medical attention for some conditions more than others.

Question 25

What are the cons of using a study?

Answer 25

• this research is less common to find (harder to find).
• clinical scenarios in research may be different from your own.

Question 26

What reference classes do we not use?

Answer 26

• incidence in the population.
• lifetime prevalence.

Question 27

what is the incidence in the population?

Answer 27

• the frequency of disease over a period of time.

Question 28

what is lifetime prevalence?

Answer 28

• the chances of developing the disease over a lifetime. will tend to be an overestimate.

Question 29

what is a consideration when using reference classes?

Answer 29

• try to find different reference classes with the useful methods above to represent maximum and minimum estimates.
• considering pre-test probabilities to the illness scripts (epidemiology).

Question 30

what can explicitly considering probability help with?

Answer 30

• better decision-making in the context of uncertainty.

Question 31

what can biases such as representativeness in place of probability distort?

Answer 31

judgements of subjective probability.