Statistics Notes

Time to event (survival) data

BMJ 1998; 317 doi: http://dx.doi.org/10.1136/bmj.317.7156.468 (Published 15 August 1998)
Cite this as: BMJ 1998;317:468.1

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  1. Douglas G Altman, heada,
  2. J Martin Bland, professor of medical statisticsb
  1. ICRF Medical Statistics Group, Centre for Statistics in Medicine, Institute of Health Sciences, Oxford OX3 7LF
  2. Department of Public Health Sciences, St George's Hospital Medical School, London SW17 0RE

    In many medical studies an outcome of interest is the time to an event. Such events may be adverse, such as death or recurrence of a tumour; positive, such as conception or discharge from hospital; or neutral, such as cessation of breast feeding. It is conventional to talk about survival data and survival analysis, regardless of the nature of the event. Similar data also arise when measuring the time to complete a task, such as walking 50 metres.

    The distinguishing feature of survival data is that at the end of the follow up period the event will probably not have occurred for all patients. For these patients the survival time is said to be censored, indicating that the observation period was cut off before the event occurred. We do not know when (or, indeed, whether) the patient will experience the event, only that he or she has not done so by the end of the observation period.

    Censoring may also occur in other ways. Patients may be lost to follow up during the study, or they may experience a “competing” event which makes further follow up impossible. For example, patients being followed …

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