A Hazard Ratio (HR) is a measure used in survival analysis to compare the risk of an event occurring at any time point in two different groups. It indicates how much more or less likely an event, such as death or disease progression, is to happen in one group compared to another. An HR of 1 means no difference between the groups, while an HR greater than 1 suggests increased risk, and an HR less than 1 indicates reduced risk. In clinical studies, such as those involving patients with cancer, the hazard ratio helps researchers understand the effectiveness of a treatment. For example, if a new drug has an HR of 0.5 compared to a standard treatment, it suggests that patients receiving the new drug have half the risk of experiencing the event compared to those on the standard treatment.