# 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