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:
Here the odds ratio is given by:
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.
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