# Born in the same minute

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understandinguncertainty.org was produced by the Winton programme for the public understanding of risk based in the Statistical Laboratory in the University of Cambridge. The aim was to help improve the way that uncertainty and risk are discussed in society, and show how probability and statistics can be both useful and entertaining.

Many of the animations were produced using Flash and will no longer work.

We recently heard about a couple who have two children of

different ages, but who were both born on the same date, and in exactly the same minute - say 7.32 am on September 30th. What is the chance of this happening to them, or to any family in this country?

Let's assume (wrongly) that the birth of a baby is just as likely at any minute throughout the year.

(In case you're interested, it's wrong firstly because it is more common for babies born in maternity units to be born during working hours rather than at night, and secondly, there is a small tendency for births to occur in the autumn rather than other seasons of the year.)

We're also going to assume that the time and date of birth

of siblings are independent of each other (Which again may well be untrue due to the habits of their parents - like one working abroad for a regular three months every year.)

But let's suppose a couple have two children.

- Either child could be born in any minute of the year.
- And there are 365.25 x 24 x 60 = 525,960 minutes in the year.
- The first child is born in one of these minutes.
- The second child is also born in one of these minutes.
- So the chance of their two children being born in the same minute is 1 in 525,960.

To see how rare this is, we need to have an idea of how many pairs of siblings there are, say, in the UK.

Well, there are about 60,000,000 people in the UK.

So if everyone was born into a family of two children there

would be around 30,000,000 pairs of siblings - of course some are in larger families and some are only children, so let's estimate 20,000,000 pairs of siblings.

If we divide this number by our 'chance of the two children being born in the same minute' number of 525,960 we get

20,000,000/525,960 ~ 40 pairs of siblings who were born in the same minute.

That's in the whole country. At the moment. All ages.

So a small club, but each case is not unique.

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