The hazard ratio (HR) is a widely used measure in clinical trials to compare the risk of events, such as death or disease recurrence, between two groups over time. It reflects the ratio of hazard rates—the instantaneous risk of the event occurring—between a treatment group and a control group. This measure provides valuable insights into the relative effectiveness of a treatment by assessing how the risk of an event differs between the two groups.

For example, in a clinical trial evaluating a new cancer drug against standard therapy, the hazard ratio quantifies the drug's impact on outcomes such as cancer recurrence or survival. A hazard ratio below 1 suggests that the treatment group has a lower risk of the event, indicating that the new treatment may be beneficial. Conversely, a hazard ratio above 1 implies a higher risk in the treatment group, favoring the control intervention. A hazard ratio of exactly 1 indicates no difference in risk between the groups.

In clinical research, the hazard ratio is particularly valuable because it considers the timing of events, not just their occurrence by the end of a study. This makes it especially useful in survival analysis, where understanding how quickly events happen is crucial for evaluating treatment effects. The HR helps researchers determine not only whether a treatment is effective but also the speed at which it influences outcomes.

Beyond oncology, the hazard ratio is applied in various fields, including cardiology, epidemiology, and public health. It is used to assess the efficacy of interventions, drugs, and treatment strategies across diverse conditions. By providing a dynamic measure of risk over time, the hazard ratio plays an important role in advancing medical research and supporting evidence-based decision-making in patient care.