maths) and you’d be daft to
gamble your money.
If a third mate joins you, the
chance that none of you shares a
birthday falls to 364/365 x
363/365 (because the third
person has a choice of two
possible shared birthdays)- a probability of 0.9918.
With every added person, the
odds of not finding a shared
birthday fall – so fast that it only
takes 23 people in one place to
have a better than 50:50 chance
of a shared birthday: a 0.5073
probability that two people share
a date (and a 0.01271 chance that
3 people do). So Francis and
Mary Huntrodds only needed a
social circle of 23 people for it to
be more likely than not that two
of them shared a birthday.
Assuming a village of 100
people, say, there was less than a
1 in 3 million chance that no
two of them shared a birthday.
This is different, of course, to
saying that Mary Huntrodds,
personally, had a better-than-
evens chance of finding a
husband with the same birthday.
But Prof Spiegelhalter didn’t pick
their gravestone at random out of
all the married couples who died
in 1680 – he noticed it because it
was special.
IMPROBABILITY MYTHS DEBUNKED
Nick Wheeler met his wife Aimee at
College in Truro. Some years later,
after getting engaged, they were
leafing through old family
photographs and discovered a
picture of them playing together as
children on a beach in Cornwall.
Apparently their families, who didn’t
know each other and hadn’t spoken,
had been on holiday in the same
place at the same time.
“This is an illustration of the law
of truly large numbers,”
says Prof DavidIf you think incredibly unlikely things
happen purely by chance, you’re
missing the point, says Professor
David Hand of Imperial College
London, author of The Improbability
Principle. “In general, chance plays
a bigger role in life than we think.
Getting that perfect job, meeting the
person of your dreams, may well
owe more to chance than to you.
“But that doesn’t mean you can’t
influence chance. To increase the
chance of meeting the person of
your dreams, you have to get outHand. “Whatever can happen,
will happen, given a large enough
number of opportunities. If you think
of the millions of families who go on
holiday, and whose children end up
building sandcastles together, it’s
not surprising that some of them
should meet again later on in life.”
A one-in-a-million event is still
very unlikely to happen to you - but
as there are 23 million families in
the UK, it’s almost inevitable that
it will happen to several of them in
any year.and meet people: sitting at home
in front of the television wondering
why you never meet anyone is not
going to do it.”23 PEOPLE
together is all that’s needed for the chance of
two sharing a birthday to be better than 50:50The chances of a
shared birthday
Here’s a pub trick: Bet your
mates that two people in the
pub share the same birthday. Of
course, if you have 367 people
in the pub, two of them must
share a birthday. But how many
people would you need to giveyourself a better than evens
chance of winning?
Your answer might be 184
(half of 366 + 1). But let’s tackle
this mathematically. It’s much
easier to work out the chances,
for a given number of people,
that any two of them don’t share
a birthday. So if there are only
two of you in the pub, the
chance you don’t share a
birthday is 364/365 or 0.9973
(leaving out leap years for easier“If you have
367 people
in the pub,
two of them
must share
a birthday”
A ‘one in a million chance’
means it’ll never happenYou can make your
own luck in lifePHOTO: francis frith collection, thinkstock x3STATISTICS