How do rate ratio and rate difference differ?

• A. Rate ratio is a relative measure of risk.
• B. Rate difference is an absolute difference in rates.
• C. Rate ratio is a relative measure of risk; rate difference is an absolute difference in rates.
• D. They are the same concept.

Answer: (C)

Rate ratio and rate difference describe two ways to compare how often an event occurs in two groups using rates per person-time.

The rate ratio is a relative measure of risk. It divides one group's rate by the other, so it expresses how many times higher (or lower) the rate is in one group compared with the other. Because you’re dividing rates, the units cancel and the result is unitless. The rate ratio tells you the strength of association and is especially useful when comparing across populations with different baseline rates.

The rate difference is an absolute measure. It subtracts one rate from the other, yielding the actual difference in events per unit time (the same rate units, e.g., per 1,000 person-years). This shows the extra burden or excess cases attributable to the exposure per unit time.

Example: if the exposed rate is 20 per 1,000 person-years and the unexposed rate is 10 per 1,000 person-years, the rate ratio is 2.0 and the rate difference is 10 per 1,000 person-years.