Which measure compares the probability of an outcome between two groups?

Which measure compares the probability of an outcome between two groups?

• A. Hazard ratio
• B. Prevalence ratio
• C. Relative risk
• D. Odds ratio

Answer: (C)

The key idea is comparing the likelihood of an outcome occurring in one group versus another, which is the realm of relative risk. Risk here means the probability that the outcome happens over the studied period. Relative risk is simply the ratio of that probability in the exposed group to the probability in the comparison group. A relative risk of 2, for example, means the outcome is twice as likely in the first group. This measure is directly interpretable in terms of probability. Hazard ratio, by contrast, looks at how quickly events occur over time (time-to-event analysis) and accounts for when events happen. Prevalence ratio compares how common the outcome is at a single point in time rather than following people to observe new cases. The odds ratio compares odds rather than probabilities and is especially common in case-control studies; it can misrepresent risk when the outcome is common, though it approximates relative risk only under certain conditions. So, when the question asks for a measure that compares the probability of an outcome between two groups, the direct and appropriate choice is the ratio of risks—relative risk.

The key idea is comparing the likelihood of an outcome occurring in one group versus another, which is the realm of relative risk. Risk here means the probability that the outcome happens over the studied period. Relative risk is simply the ratio of that probability in the exposed group to the probability in the comparison group. A relative risk of 2, for example, means the outcome is twice as likely in the first group.

This measure is directly interpretable in terms of probability. Hazard ratio, by contrast, looks at how quickly events occur over time (time-to-event analysis) and accounts for when events happen. Prevalence ratio compares how common the outcome is at a single point in time rather than following people to observe new cases. The odds ratio compares odds rather than probabilities and is especially common in case-control studies; it can misrepresent risk when the outcome is common, though it approximates relative risk only under certain conditions.

So, when the question asks for a measure that compares the probability of an outcome between two groups, the direct and appropriate choice is the ratio of risks—relative risk.