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(b) Calculate the interval for each case, and determine the
range of incubation periods by identifying the shortest and longest
incubation period.  Calculate the median incubation period. (Make a list of
the individual incubation periods from shortest to longest.  The middle
value on the list, or the average of the two middle values if there is an
even number of cases, is the median incubation period.)
(c) Table 1-12 displays data on symptom onset and incubation
period for a common source outbreak of staphylococcal food poisoning.
Table 1-12 shows the incubation periods grouped by two-hour intervals.
Both the median incubation period (3.5 hours) and the large number of cases
with illness onset between 2 and 4 hours after eating the suspect food are
consistent with staphylococcal food poisoning.
(4) Calculate attack rates.
(a) Attack rates, the percentage of ill persons, may be food or
meal-specific. For either type of attack rate to be meaningful, the
investigator must have food and/or meal histories on both ill and well
persons who were at risk of eating the suspect food or meal.
(b) Food-specific attack rates help pinpoint a suspect food
within a meal, and can support observations and conclusions on food
handling that contributed to the outbreak.  Meal-specific attack rates are
appropriate when an investigation has not pinpointed a particular meal; the
results may help focus further investigative efforts.
(c) To calculate the rates, divide the number of persons who
become ill after they ate a particular food or meal by the total number of
persons (both cases and controls) who ate that food or meal, and multiply
the results by 100.  Do the same for the persons who did not eat that
particular food or meal.  A highly suspect food or meal will have the
highest attack rate for those who ate that food or meal, and the lowest
attack rate for those who did not eat that food or meal.  The difference
between the two rates provides an easy method of comparing different meals
or different foods.
(d) When investigating a disease with a long incubation period
(e.g., hepatitis A), attack rates based on food preference rather than
actual consumption may be necessary.  A person's food preferences may be
determined by asking if, when given a choice, they always or usually eat
certain foods (e.g., raw oysters), purchase particular brand items, or dine
at a particular restaurant.
(e) Table 1-14 is an example of a food specific
attack rate analysis. Persons who reported that they ate potato salad have
a high rate of illness.  The difference in attack rates is greatest for
potato salad, which implicates this food item as the vehicle in the
outbreak.  Not all people who reported eating potato salad became ill.
Some people may not accurately remember what they ate or did not eat.  The

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