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Fink Angel
Caldari
The Merry Men
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Posted - 2008.05.01 20:54:00 -
[1]
How many people do you need to have in a room to have a 50% chance or more of at least two of those people sharing the same birthday?
No cheating by looking this one up please!
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defiler
Mad Hermit
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Posted - 2008.05.01 21:00:00 -
[2]
The answer is 23. 
After recalling this little fact I looked up this old blog, which I suspect has educated quite a few about the birthday paradox...
Mad Hermit corporation
Minding our own business since 2004 |
Fink Angel
Caldari
The Merry Men
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Posted - 2008.05.01 21:04:00 -
[3]
Doh! Wasn't expecting someone to come straight out with that! It's surprisingly low, isn't it?
If anyone fancies some mind blowing maths:
http://en.wikipedia.org/wiki/Birthday_paradox
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Tarminic
Black Flame Industries
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Posted - 2008.05.01 21:17:00 -
[4]
It's pretty simple math actually.
There are 365 potential birthdays in any given year.
The number of compared birthdays in a group of X people is (X*(X-1))/2
In a population of 23, the number of compared birthdays is (23*22)/2 = 506/2 = 253
253 is greater than 365/2, so the probability is greater than 50%
That's an oversimplification, of course...but that's basically it. 
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Tarminic - 35 Million SP in Forum Warfare
Play EVE: Downtime Madness v0.81 (Updated 4/8) |
defiler
Mad Hermit
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Posted - 2008.05.01 21:19:00 -
[5]
Well, yes, intuitively it seems rather low... Until you consider one important thing, quoth wikipedia: "The birthday problem asks whether any of the 23 people have a matching birthday with any of the others ù not one in particular". Then you might realise that there are quite a few possible pairs in a group of 23 people, and the probability of a collision rises accordingly.
The maths behind it may not be entirely obvious, but the gut feeling that there's a high chance might be. Well, at least if you're a freak and find maths interesting. 
Mad Hermit corporation
Minding our own business since 2004 |
Kyrall
Deep Core Mining Inc.
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Posted - 2008.05.01 21:45:00 -
[6]
 Originally by: defiler
Well, at least if you're a freak and find maths interesting.
Considering that it's very much human nature to be curious about things, and any reasonably intelligent human will have their curiosity aroused by some sort of puzzle, be it explicitly mathematical or not, I'd say being interested in maths doesn't make you a freak. It's just that most people don't realise they are...
- Originally by: Patch86
I have it on good authority that CCP Navigator will not just ban you, but actually eat your skin if you make a rick roll related sig. Srsly.
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Fink Angel
Caldari
The Merry Men
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Posted - 2008.05.01 21:49:00 -
[7]
Originally by: defiler
The maths behind it may not be entirely obvious, but the gut feeling that there's a high chance might be. Well, at least if you're a freak and find maths interesting.
Count me in as a freak mate, I find maths (and physics) hugely interesting. 
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Soulja
Caldari
THEM.
Phalanx Alliance
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Posted - 2008.05.01 22:01:00 -
[8]
So ho many of you share my birthday.... which is today :)
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defiler
Mad Hermit
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Posted - 2008.05.01 22:04:00 -
[9]
Originally by: Fink Angel
Count me in as a freak mate, I find maths (and physics) hugely interesting.
Yes, I sorta figured, and I hope you lot realise that I consider myself to be one.
It's fun, in a very, very brain-damaged way.
Mad Hermit corporation
Minding our own business since 2004 |
Kazuo Ishiguro
House of Marbles
Zzz
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Posted - 2008.05.01 22:17:00 -
[10]
Originally by: Tarminic
That's an oversimplification, of course...but that's basically it.
A more accurate way of thinking about it is to reason thus: take the number of ways you can place k people among 365 days without two on the same day, and divide it by the total number of ways you can arrange the birthdays when allowing collisions. The result is given by {365!/([365-k)!k!)} / {365^k}. Then 1 minus this number gives the chance that two or more people share the same birthday.
My research services
Spreadsheets: Top speed calculation - Halo Implant stats |
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Wren Alterana
Minmatar
The Baros Syndicate
Kissaki Republic
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Posted - 2008.05.01 22:46:00 -
[11]
23...wherever I go I keep seeing it. I am going crazy!!
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Kyrall
Deep Core Mining Inc.
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Posted - 2008.05.02 06:13:00 -
[12]
Originally by: Wren Alterana
23...wherever I go I keep seeing it. I am going crazy!!
23 |
Kyanzes
Amarr
Utopian Research I.E.L.
The ENTITY.
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Posted - 2008.05.02 06:34:00 -
[13]
So what are the chances that someone who posts a thread like this has his birthday on the exact same day the answerer has his, assuming they are not the same person? |
ReaperOfSly
Gallente
Lyrus Associates
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Posted - 2008.05.02 15:55:00 -
[14]
How is that mind-blowing maths? It's actually pretty basic. |
Fink Angel
Caldari
The Merry Men
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Posted - 2008.05.02 18:07:00 -
[15]
Originally by: ReaperOfSly
How is that mind-blowing maths? It's actually pretty basic.
Did you look at the Wikipedia page? It goes on to Poisson approximation and various other things, which aren't exactly "pretty basic". |
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