In a case-control study, 32 of 40 case-patients and 20 of 80 controls had stored maize indoors. What is the odds ratio for indoor storage?

1. Lay the study out as a two-by-two table before touching the arithmetic. 2. Among cases, 32 stored maize inside and 40 minus 32, or 8, did not. 3. Among controls, 20 stored maize inside and 80 minus 20, or 60, did not. 4. The odds ratio is the cross product: 32 times 60 divided by 8 times 20. 5. That is 1,920 divided by 160, which equals 12.0. 6. 3.20 divides the share of cases exposed, 80%, by the share of controls exposed, 25%, which compares exposure prevalences rather than odds. 7. 33.30 stops at the odds among controls, 20 divided by 60, read as a percentage. 8. 0.08 inverts the cross product, dividing 160 by 1,920. _Source: CDC Principles of Epidemiology in Public Health Practice (3rd ed., US federal, public domain), Lesson 3 "Measures of Risk", section Odds ratio_