# Relative Risk (RR) in Context

### You need to know the baseline risk for a disease to calculate the absolute risk

### What does a particular relative risk figure mean

#### Let's look at an example

A graphical representation of relative risk

- A relative risk [RR] of 1.0, means you are average - [there is no difference in risk between the control and experimental groups]
- A relative risk of 0.5 means that your risk is 1/2 that of average or a 50% lower risk.
- A relative risk of 1.5 means you have a 50% higher risk than average
- A relative risk of 10 means you have 10 times the average risk

Puttng relative risk into context will mean you will need to know the baseline risk of disease

- If you were told that your relative risk for multiple sclerosis was 10 - ie, you had a 10 fold increased risk for the disease above average. What does this actually mean.
- This 10 times risk does sound very ominous but is it?
- You would need to know the baseline risk of multiple sclerosis in the population you are living in.
- Thus if the baseline risk is just 0.3% or 3 in 1000 people, then your 10 times risk is now 10 x 0.3 or 3%. This means you have a 97% chance of NOT developing multiple sclerosis.

From reference [4] p 73

Understanding lifetime risk

- Let's look at a lifetime risk of breast and ovarian cancer in 100 women
- Out of these 100 women, with a 12% lifetime risk for breast cancer and a 2% risk of ovarian cancer, 12 will develop breast cancer and 2 will develop ovarian cancer.
- To apply a relative risk of 50% you need to know the lifetime risk first. Thus applying a 50% RR to the above lifetime risk, 18 out of 100 women will develop breast cancer (12 + 6) ie the 6 being the 50% increased risk from the lifetime risk of 12 and 3 will develop ovarian cancer (2 + 1).

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Statistics References