What is a hazard ratio?

Hazard ratio is commonly used when describing survival data. The hazard is the rate at which a group of animals experience an event, such as death. The hazard ratio is used to compare two groups of animals (e.g. cats with mild proteinuria vs. cats with no proteinuria). The hazard ratio is calculated from the hazard rates for the two groups of animals. A hazard ratio of 1 means that there is no difference between the two groups. A hazard ratio greater than 1 implies that the event is happening faster for the group of interest compared with the control group. A hazard ratio less than one implies that the event of interest is happening slower for the group of interest group compared to the control group.