At a banquet the attack rate was 65.4% among people who ate beef and 11.4% among those who did not, giving a risk ratio of 5.7. What does that figure mean?

1. The risk ratio is the attack rate in the exposed group divided by the attack rate in the unexposed group. 2. Here that is 65.4% divided by 11.4%, which gives 5.7. 3. A risk ratio of 5.7 means people who ate the beef were 5.7 times as likely to fall ill as people who did not. 4. A ratio of 1.0 would mean the two attack rates were equal and the exposure was not associated with illness. 5. The share of illness attributable to an exposure is the population attributable risk percent, a separate figure, which was 76.7% for the beef. 6. The probability that a result arose by chance is the p-value, obtained from a chi-square test, not from the risk ratio. 7. The number of people who ate beef is a count, and counts do not appear in a ratio of rates. _Source: CDC Principles of Epidemiology in Public Health Practice, 3rd Edition, section Step 8 Evaluate Hypotheses Epidemiologically_