Confusion about distribution of marbles among 5 persons. Example by Kahneman and Tversky

118 Views Asked by At

I came across the article

Subjective Probability: A Judgment of Representativeness Daniel Kahneman and Amos Tversky

and in particular their following example

enter image description here

I have a lot of difficulties in understanding the question

In many rounds of the game, will there be more results of type I or of type II?

My difficulties are the following. IMHO the question leads to misunderstandings. There are two possible scenarios that I'm considering.

1 case: Does the order of the numbers play a role?

If so, than, it should be equally likely to get the event $$44543 \qquad \text{or} \qquad 44444$$

2 case: Does the order of the numbers not play a role?

If so, than it is way more likely, to get the event containing three $4$, one $5$ and one $3$, than to get all five $4$s. This is because the number of strings of length $5$ containing three $4$, one $5$ and one $3$ is $$\frac{5!}{3!} = 5\cdot 4 = 20.$$

The problem is that I don't see any other scenario, but the author says

The uniform distribution of marbles II (so the event that we get five $4$s) is, objectively, more probable than the nonuniform distribution I.

I looked out what it meant for him the word "objectively" and it is intended to be as precise as a mathematical explanation should be.

1

There are 1 best solutions below

1
On BEST ANSWER

You're right in most of what you write. It's quite misleading, especially with the word “type”, which seems to suggest that all results with the same number of $3$s, $4$s and $5$s are subsumed. As you say, the uniform distribution is much less probable than all results of “type” I together.

However, it's not true that the uniform distribution and a specific instance of “type” I are equally likely. The uniform distribution has $\binom84=70$ ways to select marbles for Carl and Ed, and the other one only has $\binom85=56$. (This touches on another very sloppy aspect of the text: It says “at random” and doesn't specify a distribution. However, I think we can assume that they intended to imply “independently uniformly”.)

Too bad – someone gave me a book by Kahnemann and Tversky for my birthday; I was hoping to find the time to read it – that doesn't sound like such a promising prospect anymore now...