{
  "slug": "left-truncation",
  "term": "Left-truncation",
  "aliases": [
    "Delayed entry"
  ],
  "definition": "Left-truncation is selection into a time-to-event sample only after a unit has survived to its observation-entry time.",
  "note": "A drive already several years old at the start of a study enters the observed risk set at that age, not at birth. Drives that failed before entry are absent. This differs from right-censoring, which limits follow-up after entry, and from an unknown or missing age value. Models for a lifetime-from-birth question must account for delayed entry under their stated assumptions.",
  "status": "shipping",
  "advantages": [],
  "limitations": [],
  "related": [
    "right-censoring",
    "hazard-rate",
    "drive-day"
  ],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "citations": [
    {
      "id": "oakley-2024",
      "title": "Oakley, Forshaw, Philipson and Wilson (2024), Examining the impact of critical attributes on hard drive failure times, sections 1-3",
      "url": "https://doi.org/10.1002/asmb.2829",
      "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"
    }
  ]
}