Absolute vs Relative Risk

Absolute risk is the probability of an outcome over a specified period. Absolute risk change is a difference between risks, while relative change expresses that difference proportionally to a comparison risk. Both require a defined population, outcome, and timeframe.

Absolute risk describes the chance of an outcome over a specified period in a specified population. Absolute risk reduction subtracts the treated or exposed group's risk from the comparison risk; relative risk reduction divides that difference by the comparison risk. A reduction from two percent to one percent is one percentage point and fifty percent, respectively.

Both descriptions are valid but answer different questions. The absolute difference helps estimate how many events may be affected in a comparable population, while the relative measure describes proportional change. Neither establishes causation, an individual's exact benefit, or the importance of the outcome. Present the timeframe, uncertainty, comparator, and harms alongside the numbers, using matching denominators.

When to use it

When interpreting or communicating a percentage change in risk or another event rate, especially when the baseline is omitted.

How it can help

Show baseline and comparison risks with matching denominators, then calculate both absolute and relative changes. Include uncertainty, harms, and applicability to the decision.