Web Calculator for use with article on risk. This calculator works off-line.

Expressing the magnitude of risk of adverse effects in case-control studies:"the number of patients needed to be treated for one additional patient to be harmed" Reference: Lise M Bjerre, Jacques LeLorier BMJ 2000;320:503-506
Odds ratio = 95%confidence from to ( Use periods not commas for decimal points)
Duration of the follow-up of the case control study =
It is necessary to provide an unexposed event rate for the adverse event from another congruous source, either a randomised controlled trial or from unexposed subjects in cohort studies.
Unexposed event rate: number of adverse events = per subjects or patients
Over time interval =

Approximate 95% confidence interval from to
This CI is narrower than the true CI because it does not take into account the variability surrounding the unexposed event rate.
NNTH=number of patients needed to be treated for one additional patient to be harmed
NNTB=number of patients needed be treated for one patient to be actually protected against the adverse outcome"
(odds of outcome event v odds of no event) in intervention group
(odds of outcome event v odds of no event) in conrol group

UER=unexposed event rate
CI = Confidence interval
Programme written by DJR Hutchon (copyright)
Go to A whole range of statistical calculators Go to NNT(B) & NNT(H) for controlled trial
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This calculator is for educational use. It is believed accurate but no responsibility for accuracy of the results is accepted by the author. David J R Hutchon BSc, MB, ChB, FRCOG Consultant Gynaecologist, Memorial Hospital, Darlington, England.
Comments and suggestions are welcome and will be included in the comments section. E-mail to me at DJRHutchon@Postmaster.co.uk
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