ODDS RATIO CALCULATION for a POINT SOURCE FOODBORNE
ILLNESS OUTBREAK
The purpose of this example is to illustrate how to determine the most probable food(s)
out of many which was the most likely vehicle of an outbreak's causal organism. This
example does not illustrate any particular pathogen or processing error.
Eighty-eight people gathered at an event and shared a common meal. BBQ pork was
the most commonly eaten food, eaten by 70, but one person who did not eat it also
got sick (see Foodborne Outbreak Analysis table).
FOOD ITEM
PERSONS WHO ATE FOOD ITEM PERSONS WHO DID NOT EAT FOOD ITEM
ILL
NOT TOTAL
ILL NOT TOTAL ATTACK
ILL
ATTACK
ILL
RATE(%)
RATE (%)
ODDS
RATIO
BBQ PORK
70
15
88
79.5
1
25
26
3.8
97.2
BBQ SAUCE
59
12
71
53. 1
11
32
43
25.6
14.3
CHICKEN
14
29
43
32.6
56
15
71
78.9
0.1
ICE CREAM
50
30
80
62.5
20
14
34
58.8
1.2
A food's attack rate is found by dividing the number of people sick by the total number
of people who ate the food. Thus, the BBQ pork AR is 70/88 or 79.5%. Do the same
for each food and all diners whether sick or not until the ARs of all foods eaten at the
suspect meal are calculated. Then, plug the numbers into the Odds Ratio table and
compare the values obtained.
1. Attack Rate = ill ÷ total number who ate the food
Example for BBQ pork: 70 ÷ 88 = 79.5%
2. Calculate Attack Rates for each food.
3. Compare each AR with the highest AR, in this example the BBQ pork.
4. Calculate the Odds Ratio for each food = AxD ÷ BxC
ILL
NOT
ILL
EXPOSED (ATE FOOD ITEM)
A
B
NOT EXPOSED
C
D
ILL
NOT
ILL
EXPOSED (ATE FOOD ITEM)
70
18
NOT EXPOSED
1
25
Interpretation: people who ate BBQ pork (exposed) are 97.2 times more likely to
become sick than attendees who did not eat the pork (unexposed). The probability
for each food in an outbreak will be different, but the vehicle food(s) will almost
always have the highest probability.
5 Odds Ratio Calculation for a Point Source Outbreak
12/27/07