drive-day · shipping

Drive-day

A drive-day is one drive observed in operation for one day, used as a unit of fleet exposure.

Exposure is additive: 100 drives observed for 10 days contribute 1,000 drive-days. Count only the time covered by the selected observation rule; a fleet's end-of-period headcount is not its exposure. Daily-presence records and SMART power-on hours measure different things and must not be silently substituted.

Relations

related

afr, smart, right-censoring, left-truncation

Citations

1. Backblaze: 10 Stories From 10 Years of Drive Stats Data, sections 3-4 (accessed 2026-10-10)
2. Hesela: AFR is a rate, not a one-year failure probability (accessed 2026-10-10)
3. Hesela: Missing SMART ages and the limits of drive-age comparisons (accessed 2026-10-11)

JSON: /data/terms/drive-day.json