Birthday numbers: Sunday Salon

Cornflower and Sarah have been celebrating birthdays recently - and I'm pleased about that, because there is clear statistical evidence that the more birthdays you have, the longer a life you will lead (though there are some curious statistical outliers in February, who live around four times longer than everybody else on a per birthday basis).  But the interesting point about these two bloggers is that they share their birthday - 11 September (they even share the year, though that's not relevant today - I guess it's sometime in the late 1970s).

What interests me is the sense of surprise you have when you find someone shares your birthday; given that there are only 365 (or 366) possible dates, and billions of us on the planet, it should be quite routine.  So, question for the day - how many people do you need on a bus so there's a 50-50 chance two of them have the same birthday?  And how many before it's, to all intents and purposes, a certainty?  Answers below the heron (courtesy of Dark Puss, taken in Regent's Park, and of almost no relevance to this post except that it's a fine photograph of a noble bird).

But, as this is really a literary and bookish blog, some birthday words.  First, from Dylan Thomas, in Poem in October:

My birthday began with the water-
Birds and the birds of the winged trees flying my name
    Above the farms and the white horses
            And I rose
        In rainy autumn
And walked abroad in a shower of all my days.
High tide and the heron dived when I took the road
        Over the border
            And the gates
Of the town closed as the town awoke.

Or, in a rather different mood, this from Anthony Burgess' Earthly Powers:

It was the afternoon of my eighty-first birthday, and I was in bed with my catamite when Ali announced that the archbishop had come to see me.

 

Answers:  If there are 22 people in a room, there's a 50% chance two of them have the same birthday (Note, this is NOT the chance that one of them has the same birthday as you).  With 46 people, that becomes 95%, which is "very significant" in statistician speak.  And with 72 people, it's indistinguisahble from certainty at three decimal places - though of course, absolute certainty isn't achieved until you have 367 people (with 366, all of them could have different birthdays, though it's staggeringly unlikely).

Anyway, I hope Cornflower, Sarah, and all other birthday readers have lovely days!

Note to the mathematically inclined:  This is in essence a very simple calculation, but 29 February adds complication - either you do the calculation with 365 and ignore the extra day, or you do it with 366 and assume 29 February is just as likely as any other date.  Both assumptions are distortions, but they don't make any practical difference.

Comments

[SmallWorld]

LOVE the Dylan Thomas poem. Makes me want to go back and read some of his work...Thanks!

[Pip]

I work in a team of ten and three of us have the same birthday, with two others that week, which I think is pretty weird.

[Lindsay]

Laurie and Ann make interesting comments. I take Ann's with a pinch of salt,becasue if not one class had been without a pair of birthdays, that wd be pretty extraordinary too.

And of course, Laurie has met lots of people with the same birthday - just hasn't found out!!

[Laurie]

You know, in my 31 years of existence, I have never met anyone who shares my birthday (Aug. 7) I know lots of people on Aug. 6 and Aug. 8, but none share my birthday. Bizarre.

[Cornflower]

You've brought back memories of my teenage passion for Dylan Thomas's work (that was just a few years ago, of course ). Please could he feature again in your Friday poem posts?

[Ann (Table Talk)]

I never taught a class of children in more than thirty years of teaching without at least two of them having been born on the same day.