Re: [math-fun] Schroedinger's daughter
The following clarified for me both the case of Mary and Bartholomew. There are two ways to pick a girl from a population of two child families. 1. Pick a family at random. If it contains a girl the odds are two to one that she has a brother. 2. Pick a girl at random. Then the odds are even that she has a brother, (because there are (roughly) the same number of girls with brothers as there are girls with sisters). Or is this is the classical rather than the quantum case. dg PS. Are there really twice as many boy-girl families as girl-girl or boy-boy? It shouldn't be too hard to check. Maybe certain couples are biologically more likely to produce one sex over the other. At 03:30 PM 6/20/2006, you wrote:
OK, I thought that I understood this, but here's a variation:
A friend writes a girl's name on a piece of paper and puts it (unopened) in your pocket. A random parent of two children comes up to you and says, "I have a daughter named Mary." At this point, the probability that Mary's sibling is a girl is 1/3 (I think). However, if you look at the piece of paper, and it says "Mary", the probability changes to 1/2 (approximately).
Is this correct?
Bill C. _______________________________________________ math-fun mailing list math-fun@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun
David Gale wrote:
2. Pick a girl at random. Then the odds are even that she has a brother, (because there are (roughly) the same number of girls with brothers as there are girls with sisters).
Do you mean "pick a girl at random from a two-child family?
PS. Are there really twice as many boy-girl families as girl-girl or boy-boy? It shouldn't be too hard to check. Maybe certain couples are biologically more likely to produce one sex over the other.
With mostly anecdotal evidence, there are many families that follow the following strategy as long as they are able: (1) have a child (2) if all children are the same gender, goto 1 (3) done or with the alternative: (2)' if all children are female, goto 1 With either strategy, it is much more likely than 1/2 that a two-child family has one of each gender. --ms
I answered this once before in this thread. The answer is yes, very nearly. Since the actual distribution of births is about 51% or 52% boys, this introduces a skew into the result. The only other significant difference is the incidence of identical twins, which according to Wikipedia is about 1 or 2 per thousand births. Any deviation of individual couples from the usual distribution is so small as to be virtually undetectable. Franklin T. Adams-Watters -----Original Message----- From: David Gale <gale@math.berkeley.edu> ... PS. Are there really twice as many boy-girl families as girl-girl or boy-boy? It shouldn't be too hard to check. Maybe certain couples are biologically more likely to produce one sex over the other.
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