How would you interpret a rate ratio of 1.8 for disease X comparing group A to group B?

• A. Group A has 80% higher rate
• B. Group A has 1.8 times as many cases
• C. Group B has 80% higher rate
• D. No difference in rates

Answer: (A)

A rate ratio compares how often an event occurs in two groups per unit of time or exposure. A rate ratio of 1.8 for Group A vs Group B means the rate in Group A is 1.8 times the rate in Group B. That corresponds to an 80% higher rate in Group A (since 1.8 − 1 = 0.8, and 0.8 × 100% = 80%).

It’s about rates, not just the total number of cases. If both groups had the same amount of person-time, 1.8 times as many cases would align with this, but the key is the relative rate. A rate ratio greater than 1 indicates higher rates in Group A; less than 1 would indicate higher rates in Group B; equal to 1 would mean no difference.