Calculator for confidence intervals of odds ratio in an unmatched case control study using the null hypothesis to provide an estimate. For example groups of cases and controls studied to assess the outcome after a particular intervention. The resulting odds ratio is an estimate and is often referred to as the Peto odds ratio This calculator works off-line. (IE4)

Programme written by DJR Hutchon. Enter data into the boxes "A","B","C",and "D"
EXPTL group CTRL group
(A+C) (B+D)
OUTCOME present A = B = total A+B=
absent C = D= total C+D=

Odds ratio estimate (OR) =
95% confidence interval =
A permanent record of the analysis can be obtained by printing the page.

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.
This calculation is based on the null hypothesis and provides an estimate of the true odds ratio. Under the null hypothesis that treatment has no effect on outcome, the difference between the observed and the expected would have zero difference and variance. The advantage of this approach over direct calculation is that it will accept zero results without generating infinity. You are welcome to keep this page and use the calculator off-line. I would appreciate an E-mail if you find it useful enough to do so. E-mail to me at
Reference Chalmers et al Effective Care in pregnancy and childbirth. p63