Which metric expresses the relative instantaneous risk between two groups in survival analysis?

• A. Hazard ratio
• B. Risk ratio
• C. Odds ratio
• D. Risk difference

Answer: (A)

Hazard is the instantaneous risk of the event at a given time for individuals who have survived up to that moment. The ratio of hazards between two groups—the hazard ratio—expresses the relative instantaneous risk over time. If the hazard ratio is greater than one, the event occurs at a higher instantaneous rate in the first group; if it’s less than one, the instantaneous risk is lower. This metric is commonly estimated with Cox models and, under the proportional hazards assumption, tends to be roughly constant over time, which makes it the appropriate measure for time-to-event data with censoring. Other measures like risk ratio, odds ratio, or risk difference describe probabilities or odds over a fixed follow-up and do not capture the instantaneous, time-varying risk in the same way.