In the prison outbreak 28 East wing inmates fell ill and 129 stayed well, while 4 West wing inmates fell ill and 133 stayed well. What is the odds ratio?

1. The odds ratio is the cross product of the two-by-two table, a times d divided by b times c. 2. Here a is 28 ill and exposed, b is 129 well and exposed, c is 4 ill and unexposed, d is 133 well and unexposed. 3. The numerator is 28 times 133, which equals 3,724. 4. The denominator is 129 times 4, which equals 516. 5. 3,724 divided by 516 equals 7.2. 6. 6.10 is the risk ratio for the same data, which sits below the odds ratio because tuberculosis was common on the East wing. 7. 6.79 multiplies along the rows, 28 times 129 over 4 times 133, instead of across the diagonals. 8. 0.14 inverts the cross product, dividing 516 by 3,724. _Source: CDC Principles of Epidemiology in Public Health Practice (3rd ed., US federal, public domain), Lesson 3 "Measures of Risk", section Odds ratio_