Relative and Absolute Risks

Understanding Absolute Risk

The term absolute risk refers to the probability of a specific outcome or event happening in a specific group, usually over a defined period of time.
Absolute risk is usually stated as a percentage. For example, if the absolute risk of developing a particular disease over a ten year period is 10%, this means 10 in every 100 people (of the same population) would develop the disease over ten years.
Absolute risk should not be confused with relative risk; they measure different things and are used in different ways.

Interpreting Absolute Risk

The higher the total number of events (such as people with a disease) in a group, the higher the absolute risk of that event.
The smaller the difference between absolute risks, the less significant the difference is.
If the absolute risk is low to begin with, even a large increase in risk is not likely to result in a large absolute risk.

Understanding Relative Risk

A term called relative risk, or risk ratio, compares the risk of a specific event or outcome in one group with the risk in another group.
Relative risk is expressed as a ratio or fraction. For example, if the risk of developing a disease in a group exposed to a certain factor is 20 out of 100 and the risk in a group not exposed to the factor is 10 out of 100, the relative risk would be 20/10 or 2.0. This implies that those exposed to the factor are twice as likely to develop the disease.
A relative risk of 1 indicates no difference in risk between the two groups. A relative risk of less than 1 indicates a decreased risk, and a relative risk greater than 1 indicates an increased risk.

Interpreting Relative Risk

The advantage of relative risk is that it gives a straightforward indication of how much a particular factor increases or decreases risk.
However, the disadvantage is that it doesn’t reveal the actual chance of the event occurring. It only tells you how many times more or less likely the event is. An increased risk may not be significant if the chances of the event occurring are small to begin with.
State the magnitude of the original risk (baseline risk) and the difference between the two groups’ risks to get an accurate picture of the relative risk.