What does the odds ratio quantify in comparing two groups?

What does the odds ratio quantify in comparing two groups?

• A. The ratio of the odds of an event occurring in one group to the odds in another group.
• B. The ratio of the probabilities of the event between two groups.
• C. The difference between the probabilities of the event across groups.
• D. The correlation between group membership and the event outcome.

Answer: (A)

The concept being tested is how the odds of an event in one group compare to the odds in another group. The odds in a group are the probability of the event divided by the probability of not having the event. The odds ratio is the ratio of those odds between the two groups: OR = [P1/(1−P1)] ÷ [P2/(1−P2)]. This tells you whether the event is more or less likely in one group when you frame it in terms of odds. An odds ratio greater than 1 indicates higher odds in the first group, less than 1 indicates lower odds, and equal to 1 means no difference in odds. It’s not simply a ratio of probabilities (that would be relative risk) or a difference between probabilities (risk difference), and it isn’t a direct measure of correlation with group membership.

The concept being tested is how the odds of an event in one group compare to the odds in another group. The odds in a group are the probability of the event divided by the probability of not having the event. The odds ratio is the ratio of those odds between the two groups: OR = [P1/(1−P1)] ÷ [P2/(1−P2)]. This tells you whether the event is more or less likely in one group when you frame it in terms of odds. An odds ratio greater than 1 indicates higher odds in the first group, less than 1 indicates lower odds, and equal to 1 means no difference in odds. It’s not simply a ratio of probabilities (that would be relative risk) or a difference between probabilities (risk difference), and it isn’t a direct measure of correlation with group membership.