Which of the following is the measure of effect that represents the excess risk in the exposed group compared to the unexposed group (risk difference)?

• A. Relative risk
• B. Hazard ratio
• C. Attributable risk
• D. Population attributable risk

Answer: (C)

The concept being tested is the absolute difference in risk between exposed and unexposed groups, known as attributable risk. It represents the excess or extra risk due to the exposure and is calculated as the incidence in the exposed group minus the incidence in the unexposed group. This gives the extra cases per a defined population unit that can be attributed to the exposure itself.

This is different from relative measures like relative risk, which compare risks by ratio, or from time-to-event measures like the hazard ratio, which concern the rate of events over time. Population attributable risk goes a step further to describe the proportion of cases in the entire population that can be attributed to the exposure, taking into account how common the exposure is.