If an event occurs every three years, what is the Annual Rate of Occurrence (ARO)?

• A. 3
• B. 1/3
• C. 0.33
• D. 1/12

Answer: (B)

Annual Rate of Occurrence is the number of times an event happens in one year, so it’s the reciprocal of the interval between events. If something occurs every three years, there is one event every three years, which means the rate per year is 1/3. That value is about 0.333, often shown as 0.33 when rounded. The other options would imply more frequent events in a year (three per year) or a monthly rate (one per month, 1/12), neither of which matches “every three years.”