About risk

Disease risk is a way to describe how likely it is that a person will develop a particular disease. The chance that a person will develop a disease at some point during their lifetime is referred to as lifetime risk. Because the development of a disease can occur at different times in different people, risk is often calculated as an average among groups of people. The likelihood that a particular group of people will develop a disease compared to the average likelihood of developing the disease is called the relative risk.

Relative risk is calculated by comparing the risk in a group of people with certain characteristics against the risk of a control group (such as randomly selected individuals from the general population). For example, consider a group of people with high cholesterol, a known factor that increases the risk of developing heart disease. This group of people has a certain level of risk of developing heart disease that is higher than that of the general population – lets say a 1.5-fold higher chance. This means that 50% more people in the high cholesterol group will develop heart disease than will individuals in the general population.

As people in the two groups are monitored over time to determine whether they actually develop heart disease, we may find that 52% more people, not the 50% expected, in the high cholesterol group have developed heart disease. The difference between the actual occurrence of the disease and calculated disease risk is based on many factors – one of them being the number of individuals in each group that are being compared. The more people in each group, the more accurate the risk estimate will be. It is important to remember that risk is a statistical term that best applies to large groups of people, and that your individual risk of developing a disease and the estimated risk based on a group of people may be quite different.