If the expected frequency of a hard drive failure is once every three years, what is the ARO?

• A. 0.033
• B. 0.0033
• C. 0.33
• D. 1

Answer: (C)

Annualized Rate of Occurrence (ARO) is the expected number of times an event happens in one year. If a hard drive failure is expected once every three years, the per-year frequency is 1/3. So the ARO is about 0.333, commonly written as 0.33. This means on average, you’d expect roughly one failure every three years, i.e., about 0.33 failures per year. The other numbers would correspond to different intervals (e.g., 0.0033 ≈ one event every 300 years, 0.033 ≈ one every 30 years, 1 ≈ one every year), which don’t match the given interval.