In comparative effectiveness research, which measure expresses the ratio of hazard rates between two groups?

• A. Hazard ratio
• B. Relative risk
• C. Odds ratio
• D. P-value

Answer: (A)

When comparing time-to-event data, the key idea is the instantaneous risk of the event at a given moment, among those still at risk. The hazard ratio is the ratio of those instantaneous hazard rates between two groups. It shows how quickly events occur in one group relative to the other over the follow-up period, assuming the hazards are proportional over time. For example, an HR of 2 means the event is happening at twice the instantaneous rate at any given time in one group compared with the other. This makes it the preferred measure for comparing time-to-event outcomes in comparative effectiveness research, especially when there is censoring. Relative risk looks at cumulative risk over a fixed period, odds ratio compares odds rather than rates, and the p-value measures statistical significance rather than the size of the effect.