The chance that something will happen within a given amount of time, stated in raw numbers. In medical studies, it’s usually the chance that someone will get a disease or die of it. For example, the statement “According to the National Cancer Institute, one in 68 women will get breast cancer between the ages of 40 and 50” is a statement of absolute risk.
Deeper Dive
Absolute risk is distinct from relative risk, which is a ratio of likelihood of something occurring. Absolute risk is based on the actual numbers whereas relative risk is based on the proportional level of risk. For example, if one out of 100 infants are born with a birth defect, the absolute risk of a birth defect is 1%. If three out of 100 infants exposed to drug X are born with a birth defect, the absolute risk of a birth defect is 3%. In this example, the increased absolute risk of a birth defect when exposed to drug X is 2% since 3 – 1 = 2. However, the relative risk of a birth defect when exposed to drug A is the change from 1% to 3%, a 200% or threefold increase. That is, infants exposed to drug X are three times more likely than unexposed infants to have a birth defect. It’s just that the increase still represents only an additional 2% more infants since the baseline rate is so low.
Given the massive difference in 2% absolute risk vs. 200% relative risk in this example, it’s clear why it’s important to clarify for your audience what kind of risk you’re talking about. Ideally, you would provide both absolute and relative risk and make clear what the difference is. Sometimes one is more important than another to emphasize. Special care should be taken in headlines not to scaremonger by using relative risk when it suggests a far greater risk than absolute risk would suggest, such as in this example:
“It’s very frightening to think that exposure to drug X will triple a child’s risk of birth defects, but the absolute risk remains very low and may be worth the risk-benefit tradeoff if not taking the drug could be dangerous to the mother.”