This site is intended for healthcare professionals
General Practice Notebook
Medical search

odds ratio

FREE subscriptions for doctors and students... click here
You have 3 open access pages.

In case control studies the relative risk cannot be calculated because the incidence is not determined by the study. Thus the useful manipulation of the data is into odds ratios, thus:

  • suspected cause present: number of cases = a; number of controls = b
  • suspected cause absent: number of cases = c; number of controls = d

Here the odds ratio is given by:

  • Odds ratio (OR) = ad/cb

This equation is a result of manipulation of the relative risk equation where the assumption is made that since a and c are in fact relatively very small, the a+b and c+d parts of the equation tend to b and d respectively. It is then a matter of simplification of resulting equation.

If:

the OR =1, or the confidence interval (CI) includes 1, then there is no significant difference between the treatment and control groups

the OR>1 and the CI does not include 1, events are significantly more likely in the treatment group than the control group

the OR<1 and CI does not include 1, events are significantly less likely in the treatment group than the control group

Links:

General Practice Notebook
General Practice Notebook
The information provided herein should not be used for diagnosis or treatment of any medical condition. A licensed medical practitioner should be consulted for diagnosis and treatment of any and all medical conditions. Copyright 2016 Oxbridge Solutions Ltd®. Any distribution or duplication of the information contained herein is strictly prohibited. Oxbridge Solutions Ltd® receives funding from advertising but maintains editorial independence. GPnotebook stores small data files on your computer called cookies so that we can recognise you and provide you with the best service. If you do not want to receive cookies please do not use GPnotebook.