the comparison of one group’s probability of experiencing an event (e.g., being diagnosed with lung cancer) to a second group’s probability of experiencing that event. It is often used to describe health status following exposure to some stimulus or risk factor (e.g., lead in water) or clinical intervention. A value greater than 1 indicates the group under study has a higher probability than the control group of experiencing the event; a value less than 1 indicates the group under study has a lower probability of experiencing the event; and a value of exactly 1 indicates the two groups are equally likely to experience the event. Also called relative risk.