At a wedding, 50 of 53 cake eaters and 4 of 26 non-eaters fell ill. What share of illness among cake eaters is attributable to the cake?

1. Work out the attack rate among cake eaters: 50 divided by 53 equals 94.3%. 2. Work out the attack rate among those who did not eat cake: 4 divided by 26 equals 15.4%. 3. The excess attributable to the cake is 94.3 minus 15.4, or 78.9 percentage points. 4. Divide that excess by the attack rate among cake eaters: 78.9 divided by 94.3 equals 0.837. 5. Multiplying by 100 gives 83.7%. 6. 94.3% is the attack rate among cake eaters, not the share attributable to the cake. 7. 78.9% stops at the risk difference without dividing, and 16.3% is the complement of the correct answer. _Source: CDC Principles of Epidemiology in Public Health Practice (3rd ed., US federal, public domain), Lesson 3 "Measures of Risk", section Method for calculating attributable proportion_