Message-Id: <l03102803b34e7c17107a@[153.90.236.25]>
In-Reply-To: <l03110725b34c171facc8@[192.174.2.173]>
Date: Thu, 29 Apr 1999 15:32:57 -0600
To: "Pinhole Listserv" <pinhole@exploratorium.edu>
From: Steven Eiger <eiger@montana.edu>
Subject: Re: pinhole probability
Paul, I did the exact experiment you outline below, with the same results,
and reached the same conclusion, but now I think I should have looked at
each head (sister) of a pair, thus the two sister pairs would get counted
twice. After all, we have no clue as to which sister of a pair we are
asking, could be the first or second. The problem started with "you know a
woman." We really should be asking all the women from the coin toss,
whereas we were asking only one, if the pair were two women. The problem
lies in how we select the woman, it is not be choosing a family (pair) from
a population of families containing at least one female, but from a
population of women. I was so sure I was right before, and now I see that
I was not. This is a great problem, I certainly have enjoyed it. Steve
Eiger
>
>get a bunch of pairs of coins, siblings if you will.
>Have a partner flip all the pairs.
>
>Now have that partner select out all of the pairs that have at least one
>head showing. (Hint the head is a sister)
>(This means that 1/4 or so of the pairs that have 2 tails have been
>discarded.)
>Have your assistant line up all the pairs with the coin with a head showing
>and the other coin hidden.
>Now ask the question: In these pairs of sibling coins one was a sister,
>what was the probability that the other was a brother?
>Uncover the hidden coins.
>
>2/3 of the hidden coins are tails!
>
>Of course you say, you threw out 1/4 of the cases.
>Yes I did because that is what the problem told me to do:
>In a pair of siblings one of them is the sister, therefore throw out all
>the two brother pairs.
>
>Hands-on rules!
>
>Paul D
>
>Paul "But it is more complicated than that!" Doherty,
>Senior Staff Scientist, The Exploratorium.
>pauld@exploratorium.edu, www.exo.net/~pauld
>
>
>
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Steven Eiger, Ph.D.
Departments of Biology and the WWAMI Medical Education Program
Montana State University - Bozeman
Bozeman, MT 59717-3460
Voice: (406) 994-5672
E-mail: eiger@montana.edu
FAX: (406) 994-3190