Incidence of Coincidences!

[Plain Talker]

The young chap murdered on manor fields at the weekend was born on the same date as my father

[flamingjimmy]

I've heard two theories that I find hard to believe :

 

1. That you only need thirty people in a room to make it likelier than not that two of them will share their birthday

It's actually only 23.

 

Each person has 22 other people in the room who they could share a birthday with, counting out all the different couples so as not to double count it comes to 253 different possible pairs of birthdays. It becomes the same mathematically as if you took 506 people off the street, randomly sorted them into pairs and checked if any pairs had the same birthday. And with 253 tries it doesn't seem that weird that you'd get one really.

 

The World Cup provides a pretty decent opportunity to observe this phenomenon, as each countries world cup squad has exactly 23 players. The BBC did a quick survey of the teams and found that out of the 32 teams, 16 of them have at least two players with the same birthday. Which is exactly what is predicted by the maths.

 

In our family, there is one date which was my dad's birthday, my parent's wedding anniversary, one of our children's birthday, their twin cousins' birthday, the date we completed on our first house and last but not least, the anniversary of the death of not one, but two of our cats (different years!).

 

Now someone explain that one mathematically please!

There are seven billion people in the world, A few hundred million different families.

 

The odds of one particular date holding significance for lots of people within one family is bound to happen with hundreds of millions of goes at it. Probably quite a few times, there are probably loads of families out there with similar accounts as yours.

Edited June 23, 2014 by flamingjimmy

[aliceBB]

Using the World Cup teams as an example is flawed though. Virtually all world calss footballers were born in the same three months of the year (I forget whether it is Jan-March or Oct-Dec), so you'd expect far more coinciding birthdays anyway.

[medusa]

I went to go and buy something from someone through the forum and when I went in I thought that I knew this person's wife, but couldn't place where I knew her from. She didn't seem to remember me so I let it pass.

 

A few weeks later, when going out with my new boyfriend (met independently to these people) I found out that he knew this couple. We went round to visit this couple and coincidentally this lady's twin sister was there- who remembered working as a colleague in the same sales team in 1991.

[Alcoblog]

It's actually only 23.

 

Each person has 22 other people in the room who they could share a birthday with, counting out all the different couples so as not to double count it comes to 253 different possible pairs of birthdays. It becomes the same mathematically as if you took 506 people off the street, randomly sorted them into pairs and checked if any pairs had the same birthday. And with 253 tries it doesn't seem that weird that you'd get one really.

 

 

 

Would this mathematical rule still apply if the room was full of sextuplets, at an annual convention type thingy for instance?

 

(I know that 6 doesn't go into 23 ... maybe a sextuplet had gone to the toilet or had a dental appointment).

[flamingjimmy]

Using the World Cup teams as an example is flawed though. Virtually all world calss footballers were born in the same three months of the year (I forget whether it is Jan-March or Oct-Dec), so you'd expect far more coinciding birthdays anyway.

 

It's no-where near virtually all. You're right that there is a statistically significant but there's a decent enough spread that the maths still applies.

 

To take one team as an example I chose England (birth dates according to each player's wikipedia page):

 

January births: 2

February: 2

March: 1

April: 2

May: 2

June: 2

July: 1

August: 3

September: 1

October: 1

November: 3

December: 4

 

So the England team has players representing every month and a pretty decent spread, one that is pretty consistent with a random sampling.

[hatter]

Once when I was about 10, I borrowed a book from the school library called 'the day the ceiling fell down'.

 

Guess what had happened when I got home:suspect:

[aliceBB]

It's no-where near virtually all. You're right that there is a statistically significant but there's a decent enough spread that the maths still applies.

 

To take one team as an example I chose England (birth dates according to each player's wikipedia page):

 

January births: 2

February: 2

March: 1

April: 2

May: 2

June: 2

July: 1

August: 3

September: 1

October: 1

November: 3

December: 4

 

So the England team has players representing every month and a pretty decent spread, one that is pretty consistent with a random sampling.

 

I defer to your greater enthusiasm for such research!

 

---------- Post added 24-06-2014 at 08:13 ----------

 

Once when I was about 10, I borrowed a book from the school library called 'the day the ceiling fell down'.

 

Guess what had happened when I got home:suspect:

 

Ha! One of our children's favourite school library books was a huge non-fiction tome called 'The Worst Disasters of All Time' (cheeful, I know!). Ranged from earthquakes to volcanoes to asteroid impacts and national computer system failures. Just as well the same things didn't re-enact themselves in our house each night - although some nights it felt as though they had.

 

However, spookily, the other day I was idly thinking about how you never see dogs going for walks by themselves any more these days (used to be a common sight when I was a kid), then the same afternoon, our front door was open due to the heat and a bloody great hound bounded in off the street (scaring our cats witless, I might add) - no lead, no owner (although the latter did turn up a few minutes later, out of breath and apologetic).

Edited June 24, 2014 by aliceBB

[shaznay]

just stumbled across this thread by coincidence

funnily enough the other day my friend at work was telling me about a big coincidence that happned in her life

 

when she was about 6 years old she was in the audience at a show in Blackpool where there was a kids talent competition....the competition was won by a young lad about the same age

 

years later when my friend met and married her husband it transpired the kid who won the talent competition was to be her future husband

[SnailyBoy]

I went to the cinema, only to find that dozens of people had decided to see the same film.