rate ratio

A rate ratio compares how often something happens in one group with how often it happens in another, when both are measured as rates: cases, or deaths, per so many people per unit of time. The US Centers for Disease Control and Prevention's epidemiology course defines it as a comparison of the incidence rates, person-time rates or mortality rates of two groups, the rate for the group of interest divided by the rate for the comparison group.

The number is read against one. A rate ratio of 1.0 means the two groups had the same rate. A value above 1.0 means the group in the numerator had the higher rate, so an increased risk; a value below 1.0 means a decreased risk for that group. In the CDC's own example, cruise passengers in Alaska visited the ship infirmary for respiratory illness at 11.6 visits per 1,000 tourists a week in 1998 against 5.3 the year before, a rate ratio of 2.2, so they were more than twice as likely to fall ill.

It is close to, but not the same as, a risk ratio, which divides the proportion of people affected in one group by the proportion in another without regard to time. A trial reporting a mortality rate ratio of 0.87 is saying that the first group died at 87 percent of the second group's rate over the follow-up, which is usually reported as a 13 percent reduction.