Which statement about hazard ratio interpretation is correct?

• A. HR > 1 indicates higher hazard in the treatment or exposed group; HR < 1 indicates lower hazard; HR = 1 indicates no difference.
• B. HR > 1 indicates lower hazard in the treatment group.
• C. HR = 1 indicates no difference.
• D. HR > 1 indicates higher hazard in the treatment or exposed group; HR < 1 indicates lower hazard; HR = 1 indicates no difference.

Answer: (D)

Hazard ratio interpretation hinges on comparing instantaneous risk between two groups at any moment in time. The ratio is formed as the hazard in the treatment or exposed group divided by the hazard in the reference (control) group. If that ratio is greater than one, the treated group has a higher instantaneous risk; if it is less than one, the treated group has a lower instantaneous risk; if it equals one, there is no difference in hazard between the groups. This framing works under the usual Cox-model-like assumption of proportional hazards, meaning the relative difference in hazard stays constant over time. When the outcome is something like death or relapse, a hazard ratio above one implies a higher risk of that adverse outcome in the treatment/exposed group, a hazard ratio below one implies a lower risk, and a hazard ratio of one implies no difference. This is exactly captured by saying the hazard ratio > 1 indicates higher hazard in the treatment or exposed group; HR < 1 indicates lower hazard; HR = 1 indicates no difference.