The Birthday Paradox
Heard this on the radio recently ... How many people do you need in a room to have a 50% chance of two having the same birthday?
https://betterexplained.com/articles...thdayparadox/ 
Haven't opened the link but IIRC it's about 2530. Then at about a 95% chance it's around 40
Of course to have a 100% chance you need at least 367 people  one for each day of the year plus one more. 
Opened the link  a couple fewer than I thought. and 95% was 48.
Really cool piece of stats. Frequently used by Maths teachers to impress their classes. 
Has anyone actually tested this out for real?
I get the logic but still struggle to see that if in reality if you grouped together a number of times 23 people that reality will match the logic used. 
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I actually like the Monty Hall Problem. But it tends to be more relevant to Americans that have watched Let's Make a Deal. 
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Report back tomorrow :) 
I was on a train with 10 friends on a day out, and not one of us had the same birth month, which i thought was kind of freaky.

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I know a lot of BBS Members don't advertise their birthday, but out of 6,944 members I would think at least 25% would. (Just look at this thread for example).
That would be a data base of 1,736 known birthdays. Divide that by the magic 23 and you get 75 subsets. My fuzzy logic is that would mean at least 150 BBS members would have a birthday today (a minimum of 2 per 23 subset), yet the forums page shows there are only 5. Stick that in your pipe and smoke it! 
I worked in an office of 9 people before and 3 of us shared the same birthday.
About 20 years apart in age but unusual nonetheless. 
Hedgehog, you should never get a job in statistics.

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Very true! :) 
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Edit  beaten to it ! 
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The original problem was any two the same, not someone else the same as you which is a) very different and b) the way we intuit the (wrong) answer. 
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Remember reading this at the time, World Cup squads are perfect as there are 23 players in each one, in the 2014 the number of teams with 2 or more players with the same birthday was indeed 50%, I'm the same as you I just can't believe it would actually work out but seemingly it does
Here is the article and the proof www.bbc.com/news/magazine27835311 Quote:

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Since I didn't see an article related to the birthday stats of this year's World Cup squads, I did it myself. Of the 32 teams, 16 have two players with the same birthday. Poland takes the title with 4 sets of players with the same birthday. (all different ages, though) Portugal has three sets of players and Morocco and Brazil have two sets of players with the same birthday.

That is awesome work, exactly 50% :)
It seems incredible to have 4 on the same day. Like spinning 365 number roulette wheel 25 times and getting the same number 4 times. Though thinking of it like that I can see that it could be 50/50 to get 1 repetition in those 25 spins 
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Not totally relevant but my girlfriend shares a birthday with her Dad and I share one with my Nephew. I would have thought that would be pretty long odds. My Niece is also only 2 days from my Sister.

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I'll start if anyone wants to join 24th May 
Well I think the World Cup squad link above proves it in real life quite well
Anyway, 3rd June 
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Our office has 32 people in it with 2 pairs of identical birthdays.

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Didn't read the question properly. 
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25th October 
For those of you going to the match tomorrow, here's something you can do while you wait for kickoff. (Btw, he mentions my birthday in the video. There has to be something like a .54% chance of that happening)

Since we have another World Cup coming up, I decided to check out some birthdays. Of the 24 teams playing in the Women's World Cup coming up, 11 have players that share the same birthday. Brazil is the only country with more than one duplicate birthday with two. There are two Brazilians and two Chileans that were born on December 17th.
Throughout the whole tournament, there are three players born on 12/17/1993 and 10/2/1992. There are 28 other instances of two players having the same exact birthday (including year). https://en.wikipedia.org/wiki/2019_F...rld_Cup_squads 
3 of us had the same birthday in junior school class and 2 in my senior school one. 45 boys and 4 of us born on the same day.

For extra completeness, I checked out the birthdays for the squads in the U20 World Cup. Of the 24 of those teams, 12 had players with the same birthday. Norway, Poland, Senegal and the US had two pairs of players with the same birthday while Portugal had three sets.

Another tournament and I've checked to see how many teams have players with matching birthdays. This time, teams are allowed to select up to 26 players for the tourney. As a result, this tournament has 18 of the 24 teams with multiple players with the same birthday. North Macedonia and Poland have 4 sets of players with the same birthday. England and Turkey have 3 sets. In fact, there is one team that not only has three players that share the same birthday but those players all play for the same club team. Bit of trivia for you to guess.

Top statting FB, still find this maths hard to believe

Humans have bad intuitions regarding statistics, on average.

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My wife has a sister born on her birthday 5 years later and another sister that shares my sisters birthday, if this helps

This thread is missing Antoine Griezmann.

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1. The only requirement is that two people in the group share the same birthday. It doesn't have to be any particular day and they don't have to share it with you, just with someone else. 2. The first person in the group has a birthday, for example, 1st January. If the second person in the group is also born on 1st January then, boom, two people share a birthday and the set of two is sufficient. However, the odds of this are very low:1 chance in 365.25 or about 0.3%. (The extra .25 is for 29 Feb, which only happens every four years, of course.) 3. However, if the first person has a birthday on 1 Jan and the second has a different day (say 2 Jan) then close but no cigar. So bring in a third person. If that person has a birthday on either 1 or 2 Jan, then boom. Now the odds are actually 0.8%  I'll put the maths below. 4. If we keep repeating the process, by the time you get to 23 people it's 50% likely that two share a birthday. That's the paradox and, again, see below. 5. However, we want the odds to be 100%. So the first person in the group has a birthday on 1 Jan; the second person on 2 Jan, the third on 3 Jan... the 31st on 31 Jan, the 32nd on 1 Feb... the 60th on 29 Feb (that rare leap baby), the 61st on 1 Mar... and the 366th on 31 Dec. Whatever birthday the 367th has, they will share it with someone else in the group. The maths:

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