What is the primary interpretation of a hazard ratio of 2.0 in a survival comparison?

• A. The event rate at any time is twice as high in the exposed group compared with the unexposed
• B. The cumulative incidence is twice as high
• C. The odds of the event are twice as high
• D. The relative risk over the entire study is twice as high

Answer: (A)

In survival analysis, the hazard is the instantaneous rate at which the event occurs at a given moment, given that it has not yet happened. A hazard ratio of 2.0 means that, at any moment in time (assuming the hazards are proportional over time), the exposed group experiences twice the event rate as the unexposed group. So the comparison is about instantaneous risk, not about the total number who have had the event by a certain time.

This interpretation is different from the cumulative incidence, which is the probability of having experienced the event by a specific time and depends on how risk accumulates over the entire follow-up. It also isn’t about odds (that would be an odds ratio from a logistic model), and it isn’t the same as the overall risk ratio across the whole study, which aggregates risk over time in a different way.