Probability debate / experiment

[mortgageman]

Hello,

If there were 25 people in a room, what is the probability that at least any 2 people in that room have the same birthday anniversary?

If my sister I are are in the room, the probability is 100%!

I'm finding 50% a little hard to believe... I've been in plenty of rooms of 25 people or more and I've only met three other people, in my entire life, with my birthday and one of them is my twin (not so random).

Granted we don't always compare notes on birth-dates... But, that number sounds way too high to me...

Each time your birthday does not match another person's birthday, your chance of matching the next person's birthday increases. Right?

Not sure why... These should be independent random variables, no?

http://en.wikipedia.org/wiki/Bernoulli_process

Independence of Bernoulli trials implies memorylessness property: past trials do not provide any information regarding future outcomes. From any given time, future trials is also a Bernoulli process independent of the past (fresh-start property).
{snip}

http://en.wikipedia.org/wiki/Statistical_independence

Nateo - The question is not will at least two people share YOUR birthday - that indeed is a low probability event. The question was - what is the probability of ANY TWO PEOPLE sharing the same birthday. That is a horse of a different species.

 

[NateO]

If my sister I are are in the room, the probability is 100%!

Is this birth date, or birthday? If my brother and I are in the same room, it's 100% birthday, but not date. 2 years apart. What are the odds of brothers being born on the same date in different years.

Both in my case... The probability of your case has to be even smaller, Richard?

Although footnote 2 in Paddy's link is an interesting one in terms of birthdays being skewed...

I definitely didn't get the question right, which makes it really hard to get the answer right... So pardon the interruption, and please continue.

And, no, I'm one of the masses, born in August. While I'm sure there are a lot of people with my birthday, e.g., Bill Clinton, I've only met three others, that I can recall...

 

[Richard Schollar]

And, no, I'm one of the masses, born in August. While I'm sure there are a lot of people with my birthday, e.g., Bill Clinton, I've only met three others, that I can recall...

Nate's a Taurean?? I've always said he was full of Bull...



Just kidding Nate

 

[NateO]

Heh...

Nate's a Taurean??

Negative. I've dated a couple, though... Scary stuff!

http://en.wikipedia.org/wiki/Taurus_(astrology)

Under the tropical zodiac, Taurus is occupied by the Sun from April 20 to May 20
{snip}

My birthday makes me a Leo:

http://en.wikipedia.org/wiki/Leo_(astrology)

Under the tropical zodiac, Leo is occupied by the Sun from July 23 to August 22, and under the sidereal zodiac, it is currently from August 17 to September 16.
{snip}

Hear me roar!

 

[Richard Schollar]

Bugger - I knew one of my parents was a Taurus (one born in April, other in August) - just got it round the wrong way... ****, makes me look bloody stupid.

Still, Kristy's frequently telling us what a Pu55y Cat you are - case proved!


EDIT: the correct spelling for pu55y gets asterisked out!!! Same doesn't happen to Doggy though...

 

[NateO]

No worries, Richard, not being an expert in Astrology isn't a real short-coming, in my estimation...

That whole Leo/Taurus combo is a scary one! Been there, done that, twice! If I ask a woman what her sign is, it's simply to make sure that it's not a Taurus!!

 

[Von Pookie]

Still, Kristy's frequently telling us what a Pu55y Cat you are - case proved!

::looks around:: I do what, now?

EDIT: the correct spelling for pu55y gets asterisked out!!!

Hooray for the filters!
However, if you had used it as all one word, I believe it would have gone through: pussycat.

 

[milesUK]

Greg wrote:

This must be fundamentally flawed though... because soon you get > 1.00 probability and that can't be right...

But it can! If you are in a room of more than 365 people then the probability of one person having the same birthday as you becomes better than 1:1. It therefore follows that a point occurs (a group of 28 people) where the likelihood of ANY 2 having the same birthday is better than evens.

 

[erik.van.geit]

But it can!

from a mathematic point of view probability is "0" to "1"; "impossible" to "sure"
you can never be more sure than 100%

 

[mortgageman]

Greg wrote:

This must be fundamentally flawed though... because soon you get > 1.00 probability and that can't be right...

But it can! If you are in a room of more than 365 people then the probability of one person having the same birthday as you becomes better than 1:1. It therefore follows that a point occurs (a group of 28 people) where the likelihood of ANY 2 having the same birthday is better than evens.

I'm no bookie, but you are mixing up odds with certainty. A 1 (100%) probabiltiy is equivaltent to certainty. Greg's question was how can you be > 1 probability. The answer is (and I didn't look at his math) you can't. I sure, that a mistake was made in the math somewhere. I suspect as well that either you are guessing about the odds here or you think that better than 1:1 means >1 probability. If the latter, then clearly you must agree that it is not certain that you and someone must share the same birthday if there are 365 people in the room! If the former, than I would guess that the odds are better than 1:1 with far less than 365 people.