Having no training in probability, and having come upon the present website less than a day ago, I’m hoping someone here will be able to explain to me something basic. Let’s assume, as is apparently assumed in this post, a 50-50 boy-girl chance. In other words, the chance is one out of two that a child will be a boy—or that it will be a girl. A woman says, “I have two children.” You respond, “Boys or girls?” She says, “Well, at least one of them is a boy. I haven’t yet been informed of the sex of the other, to whom I’ve just given birth.” You’re saying that the chance that the newborn is a boy is one out of three, not one out of two? That’s what I gather from the present post, near the beginning of which is the following:
In the correct version of this story, the mathematician says “I have two children”, and you ask, “Is at least one a boy?”, and she answers “Yes”. Then the probability is 1⁄3 that they are both boys.
No. To get the 1⁄3 probability you have to assume that she would be just as likely to say what she says if she had 1 boy as if she had 2 (and that she wouldn’t say it if she had none). In your scenario she’s only half as likely to say what she says if she has one boy as if she has two boys, because if she only has one there’s a 50% chance it’s the one she’s just given birth to.
Although I don’t see what you’re getting at, shinoteki, I appreciate your replying. Maybe you didn’t notice; but about half an hour after I posted my comment to which you replied, I posted a comment with a different scenario, which involves no reference to birth order. (That is not to say I see that birth order bears on this.) I will certainly appreciate a reply, from you or from anyone else, to the said latter comment, whose time-stamp is 02 December 2012 06:51:25PM.
Having no training in probability, and having come upon the present website less than a day ago, I’m hoping someone here will be able to explain to me something basic. Let’s assume, as is apparently assumed in this post, a 50-50 boy-girl chance. In other words, the chance is one out of two that a child will be a boy—or that it will be a girl. A woman says, “I have two children.” You respond, “Boys or girls?” She says, “Well, at least one of them is a boy. I haven’t yet been informed of the sex of the other, to whom I’ve just given birth.” You’re saying that the chance that the newborn is a boy is one out of three, not one out of two? That’s what I gather from the present post, near the beginning of which is the following:
In the correct version of this story, the mathematician says “I have two children”, and you ask, “Is at least one a boy?”, and she answers “Yes”. Then the probability is 1⁄3 that they are both boys.
No. To get the 1⁄3 probability you have to assume that she would be just as likely to say what she says if she had 1 boy as if she had 2 (and that she wouldn’t say it if she had none). In your scenario she’s only half as likely to say what she says if she has one boy as if she has two boys, because if she only has one there’s a 50% chance it’s the one she’s just given birth to.
Although I don’t see what you’re getting at, shinoteki, I appreciate your replying. Maybe you didn’t notice; but about half an hour after I posted my comment to which you replied, I posted a comment with a different scenario, which involves no reference to birth order. (That is not to say I see that birth order bears on this.) I will certainly appreciate a reply, from you or from anyone else, to the said latter comment, whose time-stamp is 02 December 2012 06:51:25PM.