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  "body": "\n## Summary\n\nSurvival analysis is the part of statistics that studies the time until an event happens.\nExamples of events are death, relapse of a disease or failure of a machine.\nIts central objects are the [[survival function|QTM1HZM53Q]] and the [[hazard function|Y959NKQ3XW]] of the event time.\nIts typical difficulty is [[censoring|MY8ZZP84NP]].\nFor some subjects the event is not observed during the study, and only a lower bound for their time is known.\nStandard tools include the [[Kaplan–Meier estimator|B7AJ8RWZRC]], the [[log-rank test|F6CT1YXZ0G]] and the [[proportional hazards model|N029V3JX60]].\n\n## Explanations\n\n### L1 human\n\nA clinical trial follows patients for five years and records when each one has a relapse.\nSome patients have no relapse before the trial ends, and some move away; for them we only know that the relapse came later than their last visit.\nSurvival analysis uses these incomplete records correctly instead of throwing them away.\nIt answers questions such as \"what fraction of patients is still free of relapse after two years?\" and \"does the new drug delay relapse?\".\nIn engineering the same methods are called reliability analysis.\n",
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