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BMJ. 1995 Sep 2;311(7005):619-20.

Probability of adverse events that have not yet occurred: a statistical reminder.

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  • 1II Department of Surgery, University of Cologne, Kliniken der Stadt Köln, Germany.


The probability of adverse and undesirable events during and after operations that have not yet occurred in a finite number of patients (n) can be estimated with Hanley's simple formula, which gives the upper limit of the 95% confidence interval of the probability of such an event: upper limit of 95% confidence interval = maximum risk = 3/n (for n > 30). Doctors and surgeons should keep this simple rule in mind when complication rates of zero are reported in the literature and when they have not (yet) experienced a disastrous complication in a procedure.

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