{
  "slug": "hazard-rate",
  "term": "Hazard rate",
  "aliases": [],
  "definition": "The hazard rate is the instantaneous event rate at a given time conditional on the event not having occurred before that time.",
  "note": "For a continuous lifetime T, h(t) = f(t) / S(t), with units of inverse time, not percent probability. Specify the clock and event: device age until first failure and time since the last replacement anywhere in a cluster are different random variables. A constant hazard gives exponential survival; neither constant hazard nor independence is established by a pooled AFR.",
  "status": "shipping",
  "advantages": [],
  "limitations": [],
  "related": [
    "afr",
    "mtbf",
    "drive-day",
    "right-censoring",
    "left-truncation"
  ],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "citations": [
    {
      "id": "nist-hazard",
      "title": "NIST e-Handbook: Failure (or hazard) rate (8.1.2.3)",
      "url": "https://www.itl.nist.gov/div898/handbook/apr/section1/apr123.htm",
      "accessed": "2026-10-10"
    },
    {
      "id": "nist-exponential",
      "title": "NIST e-Handbook: Exponential distribution (8.1.6.1)",
      "url": "https://www.itl.nist.gov/div898/handbook/apr/section1/apr161.htm",
      "accessed": "2026-10-10"
    },
    {
      "id": "schroeder-2007",
      "title": "Schroeder and Gibson: Disk failures in the real world, FAST 2007, sections 2.4 and 5.3",
      "url": "https://static.usenix.org/events/fast07/tech/schroeder/schroeder_html/index.html",
      "accessed": "2026-10-10"
    },
    {
      "id": "hesela-methods",
      "title": "Hesela: AFR is a rate, not a one-year failure probability",
      "url": "https://hesela.com/analyses/afr-rate-not-probability/",
      "accessed": "2026-10-10"
    },
    {
      "id": "hesela-age-audit",
      "title": "Hesela: Missing SMART ages and the limits of drive-age comparisons",
      "url": "https://hesela.com/analyses/backblaze-age-missing-data-audit/",
      "accessed": "2026-10-11"
    }
  ]
}