What is a hazard ratio, and how does it differ from a risk ratio in cohort data?

• A. Hazard ratio compares instantaneous risk of the event at any time point between groups and accounts for censoring; risk ratio uses cumulative incidence over a fixed period.
• B. Hazard ratio is the same as the incidence rate.
• C. Hazard ratio is calculated from cross-sectional data.
• D. Hazard ratio ignores time-to-event.

Answer: (A)

Understanding how time is handled in cohort analyses is essential. A hazard ratio reflects the instantaneous risk of the event at any moment, comparing groups. It uses time-to-event data and properly accounts for censoring, meaning individuals who haven’t yet had the event or who are lost to follow-up are included without bias. The hazard itself is a rate, events per person-time, and the hazard ratio is the ratio of these hazards between groups (often assuming proportional hazards so it stays roughly constant over time).

In contrast, a risk ratio uses cumulative incidence over a fixed follow-up period. It reflects the overall probability of the event within that window and doesn’t inherently account for differing follow-up times or censoring.

So, the right statement is that the hazard ratio compares instantaneous risk at any time point and accounts for censoring, while the risk ratio uses cumulative incidence over a fixed period.