In a prison outbreak, 28 of 157 East wing inmates and 4 of 137 West wing inmates developed tuberculosis. What is the risk ratio for East against West?

1. A risk ratio divides the risk in the group of primary interest by the risk in the comparison group. 2. Risk on the East wing is 28 divided by 157, or 17.8%. 3. Risk on the West wing is 4 divided by 137, or 2.9%. 4. 17.8 divided by 2.9 equals 6.1, so East wing inmates were 6.1 times as likely to develop tuberculosis. 5. 7.00 divides the case counts, 28 by 4, and ignores that the two wings held different numbers of inmates. 6. 7.20 is the odds ratio for the same table, and 0.16 is the risk ratio computed the other way round. _Source: CDC Principles of Epidemiology in Public Health Practice (3rd ed., US federal, public domain), Lesson 3 "Measures of Risk", section Definition of risk ratio_