Simple tools for understanding risks: from innumeracy to insight

An article by:

Gerd Girenzer and Adrian Edwards

Presented by:Muhammad Husnul Khuluq

F131877

Haemoccult Test

for Colorectal

Cancer

The prevalence

of cancer was

0.3%, the

sensitivity of the

test was 50%,

and the false

positive rate was

3%

What does it mean?

DO I have a cancer?

Bad Presentatio

n

Confused/ Mispercept

ion

Inappropriate Action

Confusing numerical

representations

1. Single event probabilities

2. Conditional probabilities

3. Relative risks

Single event probabilitiesThis medicine,

however, might have 30% to 50%

chance of developing a

sexual problem

Oh, NO! I’ll have 30-50% sexual

problems.

Different reference classes (in mind) results in ambiguity

Of every 10 people who take fluoxetine, three to five will experience a sexual problem

Better Representation

Specify the reference class

Use frequency statementOR

Conditional Probabilities

The probability that a woman has a breast cancer 0.8%.

If she has breast cancer, the probability that a mammogram will show a positive result is 90%.

If a woman does not have breast cancer, the probability of a positive result is 7%.

If a woman has a positive result, what is the probability that she actually has breast cancer?

8 out of 1000 women have breast cancer.

Of these 8 women with breast cancer, 7 will have a positive result on mammography.

Of the 992 women who do not have breast cancer, some 70 will have a positive mammogram.

If a group of women have positive mammograms, how many of them actually have breast cancer?

sen

sit

ivit

ysp

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Conditional probabilities Natural frequencies

Presenting three conditional probabilities is likely becoming a trouble for understanding the

information

Natural frequencies refer only to the same class of observation. Therefore, confusion could be diminished.

Conditional probabilities VS natural frequencies

The doctors who were given natural frequencies tend to estimate more precisely than those who were given conditional probabilities

Relative Risks

Women aged over 50 years are told that undergoing mammography screening reduces their risk of dying from breast cancer by 25%

Women in high risk groups are told that bilateral

prophylactic mastectomy reduces their risk of dying from breast cancer by 80%

Of 1000 who DO NOT undergo mammography 4 will die from breast cancer

Of 1000 who DO mammography, 3 will die

from breast cancer

Of 100 who DO NOT undergo mastectomy, 5 will die from

breast cancer Of 100 who DO mastectomy, 1 will die from breast cancer

Absolute risks reduction

People tend to misunderstand the relative risks like (for the 1st case), of 1000 who do the test, 25% will save and 75% others will die from breast cancer

X

Confusing Factors and How to Avoid it

1. Reference classFor each case of confusing numerical representation, misunderstanding of the reference class always exists

Clarify the reference class first.OR

Use transparent representations like natural frequencies

2. FramingNegative framing (negative-oriented information) tend to worry the patients very much and negate to undertake the treatment

Use negative frame for communicating clinical risk that the patient might still choose whether to take or not

Use positive frame to persuade patient for risky treatment

3. ManipulationThe use of verbal explanation tend to confuse the patients

Manipulate the way of representing statistical information.

The use of pictorial representation or chart might be helpful for patients to understand the information

Strength Weakness

The article is well structured, completed with examples, and charts

The important point are highlighted, so people will not loose the content

Less explanation of the example given. So it becomes less

readable for people who are not familiar with the terms

No explanation why statistical innumeracy is ignored (whereas in the beginning they it is given

as a problem)

Thank you very much for your attention