1 Ch 11 Estimating Risk: Is There an Association? Table 11-1 A hypothetical investigation of a foodborne disease outbreak The suspect foods were identified The attack rate (or incidence rate) of the disease was calculated for: those who ate the food (exposed) those who did not eat the food (nonexposed)

2 Is There an Association? Table 11-2 Column C - the ratio of the attack rate in those who ate each food to the attack rate in those who did not eat the food Column D - an alternate approach subtract the risk in those who did not eat the food from the risk in those who did eat the food the difference represents the excess risk in those who were exposed

3 Is There an Association? The ratio of the risks (or of the incidence rates): Disease risk in exposed Disease risk in nonexposed The difference in the risks (or in the incidence rates): Disease risk in exposed - Disease risk in nonexposed

4 Is There an Association? Table 11-3 The difference in incidence rates: 30% The ratio of the incidence rates: 4.0 vs. 1.5

5 Relative risk The ratio of the risk of disease in exposed individuals to the risk of disease in nonexposed individuals Disease risk in exposed Relative risk = Disease risk in nonexposed

6 Relative risk Table 11-4 If the relative risk is equal to 1 - no evidence exists for any increased risk in exposed individuals If the relative risk is greater than 1 - this is evidence of a positive association, and may be causal If the relative risk is less than 1 - evidence of a negative association, and it may be indicative of a protective effect can be observed in people who are given an effective vaccine ("exposed" to the vaccine)

7 Relative risk Table 11-5 Risk calculations in a Cohort Study Incidence in exposed Relative risk = Incidence in nonexposed a / (a + b) = c / (c + d)

8 Relative risk Table 11-6 Smoking and CHD Incidence in exposed Relative risk = Incidence in nonexposed 28 = =

9 Relative risk Table 11-7 The first 12 years of the Framingham Study relating risk of coronary disease to age, sex, and cholesterol level The relation of risk to cholesterol level: men vs. women The incidence rate of 38.2 in younger men with low cholesterol levels was assigned a risk of 1.0; these subjects are considered "nonexposed."

10 Relative risk Figure 11-4 Data on 2,282 middle-aged men followed up for 10 years in the Framingham Study and 1,838 middle-aged men followed up for 8 years in Albany, New York The data relate smoking, cholesterol level, and blood pressure to risk of myocardial infarction and death from CHD A value of 1 was assigned to the lowest of the risks in each of the two parts of the figure, and the other risks are calculated relative to this value

11 Relative risk Figure 11-4 Note: the risk is higher with high cholesterol levels, and that this holds both in smokers and in nonsmokers (although the risk is higher in smokers even when cholesterol levels are low) Thus both smoking and elevated cholesterol levels contribute to the risk of myocardial infarction and death from CHD Blood pressure and smoking