Archive for 2011/08/16

The probability of thing(s)

It is so easy to think we grasp probability and on a logical class room level we might but in real life situation, in comes our illogical brain and messes with us. This example is taken From The Drunkard’s walk (

Imagine Linda, 31 years old, single, outspoken and very bright. In college she majored in philosophy. while a student, she was deeply concerned with discrimination and social justice and participated in anti-nuclear demonstrations.

Now rank the probability of the following statements on a scale 1-8 based on their probability where 1 is the most probable and 8 is the least probable.

Linda works in a bookstore and takes yoga classes
Linda is an ensurance sales person
Linda is active in the feminist movement
Linda is a bank teller and is active in the feminist movement
Linda is a phyciatrical social worker
Linda is a teacher in an elementary school
Linda is a member of the league of feminine voters
Linda is a bank teller

Now look at your list before scrolling down.

If you are like the average person you would rank the scenario

Linda is a bank teller and is active in the feminist movement

as more probable than this scenario:

Linda is a bank teller

Look again.

If you were at a math class these scenarios would read

Linda is a bank teller = A

Linda is active in the feminist movement = B

This would lead to statement “Linda is a bank teller and is active in the feminist movement” = A +B

So, how could A+B be more probable than A alone? It’s not but our minds are playing with us. Giving that personal description, we get an image of a feminist activist and that seems so probable that we forget about maths and logic. And if we look again, even if Linda was a feminist in her youth, this might not be true anymore.

You might not have fallen for the little trick here, but most people do. They search for patterns and a pattern is often more appealing to our brains than the logical rules of probability. I cannot help thinking about estimation poker when I see this example. Also, we humans love story telling and by just adding another statement to the description of Linda. By describing her using two words she become more probable than if described using one word.

What do you think?

Categories: planning