Infirmary visits for acute respiratory illness ran at 11.6 per 1,000 tourists per week in 1998 against 5.3 in 1997. What is the rate ratio?

1. A rate ratio divides the rate for the group of primary interest by the rate for the comparison group. 2. 1998 is the year of interest, so put 11.6 on top and 5.3 underneath. 3. 11.6 divided by 5.3 equals 2.19, which rounds to 2.2. 4. Passengers in 1998 were therefore more than twice as likely to visit the infirmary as those in 1997. 5. 6.30 is the difference between the two rates and 16.90 is their sum, so neither is a ratio. 6. 0.46 divides 5.3 by 11.6, comparing 1997 with 1998 instead. _Source: CDC Principles of Epidemiology in Public Health Practice (3rd ed., US federal, public domain), Lesson 3 "Measures of Risk", section Rate ratio_