What is an odds ratio?
An odds ratio is a way to compare the probability of something happening in one group versus the probability of the same thing happening in another group. An odds ratio of 1 between two groups means that the probability of the event (e.g. developing a disease or a treatment working) is the same for both groups. An odds ratio greater than 1 means that the event is more likely in the first group, while an odds ratio less than 1 means that the event is less likely in the first group. When comparing probability across more than two groups, one of the groups is chosen as the reference category, and the odds ratio is calculated for each group compared with the reference category.