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odds ratio
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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
Last reviewed 08/2020
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