# How to Calculate Relative Risk: 3 Steps (with Images)

Relative risk is a statistical term used to describe the risk of a certain event happening in one group against another. It is commonly used in epidemiology and evidence-based medicine, where relative risk helps to identify the risk of developing a disease after an exposure (eg, a drug/treatment or environmental exposure) versus the risk of developing the disease in the absence of exposure. This article will demonstrate how to calculate this relative risk.

## Steps #### Step 1. Draw a 2x2 table

A 2x2 table is the basis of many epidemiological calculations.

• Before you draw a 2x2 table, understand the variables:

• A = The number of people who have both exposure and development of the disease
• B = The number of people who have the exposure but have not developed the disease
• C = The number of people who were not exposed but developed the disease
• D = The number of people who have neither exposure nor development of the disease.
• Let's do an example 2x2 table.

• One study takes 100 smokers and 100 non-smokers and follows them to see the development of lung cancer.
• Early on, we can fill part of the table. The disease is lung cancer, the exposure is smoking, the total numbers for each group is 100, and the total for all people studied is 200.
• At the end of the study, they found that 30 smokers and 10 non-smokers developed lung cancer. Now we can fill in the rest of the table.
• Since A = number of exposed people who have the disease (ie smokers who have cancer), we know it is 30. We can calculate B simply by subtracting A from the total: 100 - 30 = 70. Likewise, C is the number of non-smokers who have cancer, which we know to be 10 and D = 100 - 10 = 90. #### Step 2. Calculate the relative risk using the 2x2 table

• The general formula for the relative risk, using the 2x2 table is:

• RR=A/(A+B)C(/C+D){displaystyle RR={frac {A/(A+B)}{C(/C+D)}}}

• Podemos calcular o risco relativo usando nosso exemplo:

• RR=30/(30+70)10/(10+90)=0, 30, 1=3{displaystyle RR={frac {30/(30+70)}{10/(10+90)}}={frac {0, 3}{0, 1}}=3}
• Portanto, o risco relativo do câncer adquirido com fumantes é 3. #### Step 3. Interpret the relative risk result

• If the relative risk = 1, then there is no difference in risk between the two groups.
• If the relative risk is less than 1, then there is less risk in the exposed group than in the unexposed group.
• If the relative risk is greater than 1 (as in the example), then there is a greater risk of the exposed group relative to the unexposed group.