What does the risk difference represent?

• A. The ratio of risks.
• B. Difference in hazards.
• C. Probability of exposure.
• D. Absolute difference in risk between exposed and unexposed.

Answer: (D)

The risk difference is an absolute measure of how much the risk of the outcome changes when exposure is present, comparing two groups directly. It is calculated as the risk in the exposed group minus the risk in the unexposed group.

Interpreting it in practical terms: if 6% of the exposed develop the outcome and 2% of the unexposed do, the risk difference is 4 percentage points. This means the exposure adds 4 percentage points to the risk, an absolute increase in risk. If the difference were negative, it would indicate a protective effect of exposure, reducing risk by that amount.

This is different from a ratio of risks (risk in exposed divided by risk in unexposed) and different from hazards, which are time-to-event concepts. It’s about the actual, concrete difference in probability between the two groups, expressed in the same units as the outcome risk (often percentage points).