A lоng-term study is exаmining the risk оf cаr аccidents in thоse who own a cell phone versus those who do not. The study asks participants to record the amount of time they spend in the car over the course of the study. The study finds that those who own a cell phone got in 23 accidents over the course of 5,101 hours driving (40.5 accidents/10,000 person-hours of observation); whereas among those who did not own a cell phone, they had 4 accidents over 5,981 hours of driving (6.7 accidents/10,000 person-hours of observation). The measure of risk calculated by creating a ratio of these two rates is known as: