Geoscience Reference
In-Depth Information
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EXAMPLE 8.6
Problem:
In an outbreak of gastroenteritis among attendees of a corporate picnic, 99 persons ate
potato salad, and 30 of those developed gastroenteritis. Calculate the risk of illness among the per-
sons who ate the potato salad.
Solution:
Numerator = 30 persons who ate potato salad and developed gastroenteritis
Denominator = 99 persons who ate potato salad
10
n
= 10
2
= 100
Risk (“food-specific attack rate”) = (30/90) × 100 = 0.303 × 100 = 30.3%
Two example fractions used in incidence proportion problems are
Number of womeninFraminghamStudy whodieddthrough last year fromheartdisease
Number of womeninitiallyenrolledinFramingham Study
Number of womeninFraminghamStudynewly diagnosed with heartdiseaselastyear
Number of womeninFraminghamStudy without heartdiseaseatbeginning of same year
8.3.1.1 Properties and Uses of Incidence Proportions
Incidence proportion is a measure of the risk of diseases or the probability of developing a particular
disease during a specified period. As a measure of incidence, it includes only new cases of disease
in the numerator. The denominator is the number of persons in the population at the start of the
observation period. Because all of the persons with new cases of disease (numerator) are also repre-
sented in the denominator, a risk is also a proportion. In the outbreak setting, the term
attack rate
is
often used as a synonym for risk. It is the risk of getting the disease during a specified period, such
as the duration of an outbreak. A variety of attack rates can be calculated.
Overall attack rate
is the
total number of new cases divided by the total population. A
food-specific attack rate
is the number
of persons who ate a specified food and became ill divided by the total number of persons who ate
the food, as illustrated in the previous potato salad example. A
secondary attack rate
is sometimes
calculated to document the difference between community transmission of illness vs. transmission
of illness in a household, barracks, or other closed population. It is calculated as
Number of cases among contactsofprimary cases
Total numberofcontacts
×10
n
DID YOU KNOW?
The denominator of an incidence proportion is the number of persons at the start of the
observation period. The denominator should be limited to the “population at risk” for devel-
oping the disease (i.e., persons who have the potential to get the disease and be included in
the numerator). For example, if the numerator represents new cases of ovarian cancer, the
denominator should be restricted to women, because men do not have ovaries. This is easily
accomplished because census data by sex are readily available. In fact, ideally the denomina-
tor should be restricted to women with ovaries, excluding women who have had their ovaries
removed surgically (often done in conjunction with a hysterectomy), but this is not usually
practical. This is an example of field environmental health practitioners doing the best they
can with the data they have.
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