Transcript Risk-baconsandwiches - Motivate

Eating bacon
sandwiches is
bad for you!
Is it?
Evaluating the risk
Eating bacon
sandwiches is
bad for you!
If you eat a bacon sandwich
every day, your risk of getting
bowel cancer at some point in
your life is increased by 20%.
Eating bacon
sandwiches is
bad for you!
If you eat a bacon sandwich
every day, your risk of getting
bowel cancer at some point in
your life is increased by 20%.
Sounds
bad! What
do you
think?
Eating bacon
sandwiches is
bad for you!
If you eat a bacon sandwich
every day, your risk of getting
bowel cancer at some point in
your life is increased by 20%.
Before we can
decide how bad
this is, we need
to know what
this is 20% of.
Sounds
bad! What
do you
think?
But why does it
matter what
it’s 20% of?
20% is 20%, isn’t it?
Colour the right
number of squares on
your worksheet.
What’s 20% of 100 people?
What about 20% of 10 people?
Or 20% of 150 people?
20% of 100 people = 20
20% of 10 people = 2
20% of 150 people = 30
20% of 100 people = 20
20% of 10 people = 2
20% of 150 people = 30
2 is a lot
fewer people
than
20 or 30
The missing information …
We need to know how many people will get
bowel cancer even if they don’t eat bacon
sandwiches to know whether we should
worry about a 20% increase or not.
If there’s a large risk anyway, then a 20%
increase will be a lot more people.
If there isn’t a large risk anyway, then a 20%
increase will be a much smaller number of
people.
The missing information …
We need to know how many people will get
bowel cancer even if they don’t eat bacon
sandwiches to know whether we should
worry about a 20% increase or not.
If there’s a large risk anyway, then a 20%
increase will be a lot more people.
If there isn’t a large risk anyway, then a 20%
increase will be a much smaller number of
people.
The missing information …
We need to know how many people will get
bowel cancer even if they don’t eat bacon
sandwiches to know whether we should
worry about a 20% increase or not.
If there’s a large risk anyway, then a 20%
increase will be a lot more people.
If there’s only a small risk anyway, then a 20%
increase will be a much smaller number of
people.
The risk that a person will get
bowel cancer at some point in
their life is about 1 in 18, which is
roughly 5%.
This means that even if they
never eat a bacon sandwich about
5 people in every 100 will get
bowel cancer.
20% of 5 people is 1 person.
So an extra person can be
expected to get bowel cancer if
100 people eat bacon sandwiches
throughout their lives.
It is this risk – the absolute risk of getting
bowel cancer – which is increased by 20% if
someone eats a bacon sandwich every day.
The risk that a person will get
bowel cancer at some point in
their life is about 1 in 18, which is
roughly 5%.
This means that even if they
never eat a bacon sandwich about
5 people in every 100 will get
bowel cancer.
20% of 5 people is 1 person.
So an extra person can be
expected to get bowel cancer if
100 people eat bacon sandwiches
throughout their lives.
On your worksheet
colour a random 5% of
smiley faces on each
grid to show people
who will get bowel
cancer regardless of
eating bacon
sandwiches
5% of 100 is 5
5% of 60 is 3
5% of 140 is 7
Using a different colour
add the extra cases of
bowel cancer which
would occur if all these
people ate bacon
sandwiches every day
An extra 20% of 5 is 1,
giving a total of 6.
An extra 20% of 3 is 0.6,
giving a total of 3 or 4.
An extra 20% of 7 is 1.4,
giving a total of 8 or 9.
The missing information …
A 20% increase sounds a lot, doesn’t it.
But because it’s 20% of a small absolute risk, it
actually means that we should expect just one extra
person to get bowel cancer in every 100 who eat
bacon sandwiches every day.
What do you think? Will this stop you eating bacon
sandwiches?
Motivate Project
Millennium Mathematics Project
University of Cambridge
We are grateful to the
Wellcome Trust
for funding this project.
© University of Cambridge