# Calculating the attack rate and risk ratio

Let's look now at Table a.

Using Table a, let's calculate the attack rate in the exposed group (these are the people who smoked).

Attack rate (exposed Gp) = No. of people who got cancer/ No. people in the group

= 20/100 = 0.2

Now let's calculate the attack rate in the unexposed group (these are the people who did not smoke).

Attack rate (unexposed Gp) = No. people who got cancer/ No. people in the group

= 1/100=0.01

Calculating now the relative risk (RR)

RR = attack rate exposed group/attack rate unexposed group

= 0.2/0.01=20

So what does a RR of 20 mean?

This means that people who are exposed to smoking are 20 times more likely to develop cancer than people not exposed to smoking.

So to recap

A RR of 1 – means the risk of disease is similar in the exposed group and the unexposed group

RR > 1 – means that risk of disease is greater in the exposed group than the unexposed group

RR < 1 – means that risk of disease is less in the exposed group than the unexposed group

Watch the video below for a worked example of the calculation.