Would you disagree?
no i was wrong
on your original 33% 50% with one boy and then the first is a boy.
Your answers are correct. The gender of the next born child is unaffected by the gender of the previous child so it will always be 50/50.
Chat Forum
[closed]
Boys/Girls... probability ponderer
-
no i was wrong
on your original 33% 50% with one boy and then the first is a boy.
Your answers are correct. The gender of the next born child is unaffected by the gender of the previous child so it will always be 50/50.Posted 1 year ago # -
(seems I dont do maths or logic) :
so if IGM can make that 50% argument for the eldest I think I can make it for the youngest, too
If so, it doesn't matter which the 1st mentioned kid is, the answer is 50%
So why isn't it 50% anyway ? 
Posted 1 year ago # -
'Cos it isn't. It just isn't.
Posted 1 year ago # -
By the way if you like that one...
If a man walks up a hill, taking all day to do so, and camps at the top before walking down the next day, is there a point where he is in the same position as he was exactly 24 hours earlier?
You may all get the answer but it normally starts arguments.
If you care.
Posted 1 year ago # -
is the mountain on a conveyor belt?
Posted 1 year ago # -
No but it comes with a crown race...
Posted 1 year ago # -
aaaaggghhhhh, FFS!
OK, how long is "all day"
how long does he camp for ?
How long does it take him to walk down ?(trick questions?):
same route down ?
what do you mean by position ?Posted 1 year ago # -
(seems I dont do maths or logic) :
so if IGM can make that 50% argument for the eldest I think I can make it for the youngest, too
If so, it doesn't matter which the 1st mentioned kid is, the answer is 50%
So why isn't it 50% anyway ?
ok four outcomes M M , M F, F M , F F
with one being female we get M M , M F and F M so 1 in 3 of two males
If we know the eldest is male we have only M M and F M as options so 50/50 of two males
We reduce the number of combinations so change the odds.24 hours assuming lots of things that just wont happen in the real world yes but not really as thye will be facing another direction unless they walk backwards
Posted 1 year ago # -
I'm sure there's some hormonal thing that ups the chances of the second being a girl if the first is.
Stat that
Posted 1 year ago # -
well, yeah junkyard
but if you tell me that one of your kids is male and I say "If he's the eldest, there's a 50% chance you have 2 boys and if he's the youngest, thers a 50% chance you have 2 boys" how am I wrong ?
(I need something verbal to back up the maths because I'm just not feeling it for 1/3)Posted 1 year ago # -
You're not wrong.
But if I don't tell you he's the oldest or youngest the odds are 33%.
Right about now you will be saying how can they be anything other than the oldest or youngest - but the knowledge about the family makes a difference to what you can say about them.
Posted 1 year ago # -
but if you tell me that one of your [two] kids is male
and I say "If he's the eldest, there's a 50%
chance you have 2 boys and if he's the
youngest, thers a 50% chance you have 2
boys" how am I wrong ?You're not. You'd be correct.
Posted 1 year ago # -
Is the answer baby robin?
Posted 1 year ago # -
Sorry folks, I had planned on responding prior to this, but seeing as I got dragged away from t'internerd I couldn't get back on..
GrahamS, to answer your question, I am not sure as I wasn't the original one to ask the question, I heard it on Radio 4's More or Less, but it looks like it's made it into the New Scientist as well... it was originally asked at a maths/puzzle convference in the US.
However, I would guess that the Tuesday only relates to a the known boy.
Looking through the answer myself, I can see how the maths works out. Essentially the extra information you now have means that the order of a B B family is important with BTu B being different to B BTu unlike in the original where B B was just a single result.
Unlike Junkyard however, I think that this result is followed in the real world and would be backed up empirically. You should be able to reproduce this in a spreadsheet in just the same way as GrahamS did for the original.
As for the 50%ers... hehehe... now, I have these three doors, behind one of them is a stack of cash... ;o)
Posted 1 year ago # -
Unlike Junkyard however, I think that this result is followed in the real world and would be backed up empirically.
I dont agree but the reason the probabilities change is because we are only sampling a percentage of the TOTAL possible outcomes and the odss within that subset are as stated the actual odds have not altered.
MM MF FM FF
is reduced to MM MF FM hence the 1 in 3 as we ignore 25% of reality. I suspect the Tuesday does the same However I cannot really see how the dat affects anything in this example. In the first example the child must be born on a day being told which day it is has not altered anything in the real world. I agree them aths /spreadsheet will hold but it is not "real". It will not be observed by actually monitoring of reall births IMHO.Posted 1 year ago # -
I agree them aths /spreadsheet will hold but it is not "real"
What if you used the spreadsheet to count a "real" collection of say 10,000 people that actually met the criteria? Would it become real then?
It will not be observed by actually monitoring of real births
As pointed out, real births are no where near this simple. The problem is set in a statistically perfect world, where every birth has a 50:50 chance of being either one boy or one girl and there are no troublesome factors like genetic predispositions, environmental influence, multiple births, IVF, XXY "girlboys", etc etc
At best you might see a rough approximation of this in the real world.
But yes, I'm reasonably convinced that if you monitored births within this imaginary construct then you would see the figures discussed.
Posted 1 year ago # -
What if you used the spreadsheet to count a "real" collection of say 10,000 people that actually met the criteria? Would it become real then?
That is my point it would be real but would come out as 33% not 13/27.
Yes my assumptions are made in a statistically perfect world as well.
Essentially you are arguing that the day your child is born on affects the likelyhood of their gender and i say that is B0ll0cks for. The gender of the sibling alters the likelyhood of their gender I cannot see how day of birth does this as all children are born on a day] and i has has no bearing on likelyhood of gender.
I am astarting to get paranoid that I am doing a Smee here though
I need to do asomemaths n this have we just reduced the set of all possibilities even further so that we have 13/27 but for less than 75% of the total births?Posted 1 year ago # -
I understand your confusion Junkyard, and share it slightly.
I think the key is that "The gender of the sibling alters the likelyhood of their gender" isn't really true. It just alters the likelihood that we are dealing with a child in a particular sub-group.
So the same applies to adding the day of the week information. It doesn't really mean that "you are arguing that the day your child is born on affects the likelyhood of their gender " it just means the sub-groups we must consider have changed and so have the probabilities that you are dealing with a child in a given sub-group.
I think.
If I get time I might do a spreadsheet...
Posted 1 year ago # -
yes get the first bit we have removed one of the four posiibilitie shence 1 in 3 with you re sub set.
I agree with the next bit as well re the maths but I cannot believe thtat day of the week actually alters the probability of the gender after all in the first one the baby was born on a day of the week just because we now know which one I cannot see hwy th eprobabilities order.
I suspect the maths is correct but I dont agree with the conclusion reached that is that the odds have changed - your spreadsheet will show it as you describe no need to do this for me but if the inner Geek feels the need dont let me stop you Posted 1 year ago # -
My Inner Geek is strong:
http://spreadsheets.google.com/ccc?key=0Ai9dvO8nev8icF9INW8yU2VwM1BOeFBoQjFDamItRGc&hl=en_GB
The spreadsheet shows 1,000 pairs of siblings. It is 50:50 whether each is boy or girl and 1 in 7 for the day of the week they were born.
It should re-randomise them every time you refresh it.
I'm currently seeing: 119 pairs where at least one child was a Boy born on a Tuesday and of that 54 (45.9%) had a brother.
Obviously it would be more accurate with a larger sample size, but I think it demonstrates the point
Posted 1 year ago # -
bows down at the alter of stats but you have linked to the old boy /girl one not the new one...unless i am as bad at computing as I am at getting this.
Posted 1 year ago # -
bows down at the alter of stats but you have linked to the old boy /girl one not the new one...unless i am as bad at computing as I am at getting this.
Posted 1 year ago # -
Try Sheet Two (tab at the bottom of the spreadsheet)
Posted 1 year ago # -
(did it work? did you see?)
Posted 1 year ago # -
sticks head in sand and refuses to play !
Yes and I dont fault the maths but I dont understand what has been removed to increase the odds. With the first example we loose FF hence the change what is it in the second that has been removed by knowing birth day? Will have a proper look / think tomorrow and you can help lead me to the light
Posted 1 year ago # -
graham, can you easily add a cell or two to count how many (or, rather %) boys have:
older or younger brother
older or younger sister
Posted 1 year ago # -
Yep, I'll do it tomorrow. But is should be around 50% for both.
Posted 1 year ago # -
Done, as expected:
Boys with brothers ~ 25%
Boys with sisters ~ 50%
All girls ~ 25%Pretty much as per the original problem.
Posted 1 year ago # -
But is should be around 50% for both
as expected (25%, 50%)
Posted 1 year ago # -
Yeah, yeah, I misread your query yesterday.
Anyway, does that help?
Posted 1 year ago # -
(no
) cheers
Posted 1 year ago # -
Lol, oh well I tried. At least it demonstrates the maths is "real world"
Posted 1 year ago # -
scaredypants... try looking at it like this...
If you assume he is the eldest then there is a 50% chance the younger child is a boy
If you assume he is the youngest then there is a 50% chance the older child is a boy
That gives an overall 25% chance that the both children will be boys, which agrees with GrahamS's spreadsheets above, but this is the chance for 2 boys in the population as a whole.
By then using the knowledge that the family has to have at least one boy, that means that we can ignore all those families with 2 girls, which is 25%.
If we take the whole sample to be 100 families, this means that the number of families with 2 boys is 25/(100-25) = 1/3 or 33%.
Any better?
Posted 1 year ago # -
Funkynick - I'm way beyond help
I can "see" the statistical answer but I can't get it to agree with my own special logic*
*which would say that what you said about probabilty above should, if correct, also apply to him having an older or younger sister - which would leave us with a 33% chance of his sibling being neither a girl nor a boy
Posted 1 year ago # -
Junkyard... you are not losing anything to increase the odds, nor is the day of the week having any effect on the likelihood of a child being born one sex or another... it's simply a matter of probability.
What happens is this...
In the previous problem, where we just know that one child is a boy, which gives the answer of 33%, we only know the following:
There are 2 children
One is a boy
The birth rate is an idealised 50:50 splitThis means it only allows us to have four choices of how the family is made up:
GG BG GB BB
And so losing the GG option as we know there is at least one boy, gives us the 33% answer.
However, in the new problem, where we also know the boy was born on a Tuesday, this gives us extra information and so an extra option... so that now we have the following possibly options:
GG BTuG GBTu BTuB BBTu
Once again losing the GG option leaves us with the remaining 4, and the maths continues as posted on the first page, giving us the answer of 13/27.
So, by being given the additional information it means that the order of the birth for the two boys options becomes important, whereas previously, the order for the two boys was unimportant as all we knew was that there was at least one boy.
All this means is that as we get more and more information about the children involved, then the closer and closer the answer gets to being the 50% that a lot of people thought was the answer to the original question.
Any clearer?
Posted 1 year ago #
Topic Closed
This topic has been closed to new replies.
