Geoscience Reference
In-Depth Information
TABLE 8.9A
General Format and Notation
for a Two-by-Two Table
Ill
Well
Total
Exposed
a
b
a
+
b
=
H
1
Unexposed
c
d
c
+
d
=
H
0
Total
a
+
c
=
V
1
b
+
d
=
V
0
294
TABLE 8.9B
Incidence of
Mycobacterium tuberculosis
Infection Among Congregated, HIV-Infected
Prison Inmates by Dormitory Wing—South
Caroline, 1999
Developed Tuberculosis?
Yes
No
Total
East wing
a
= 28
b
= 129
H
1
= 157
West wing
c
= 4
d
= 133
H
0
= 137
Total
32
262
294
Source:
Data from CDC (2012) and McLaughlin et al. (2003).
Attack rate for exposed =
a
/(
a
+
b
)
Attack rate for unexposed =
c
/(
c
+
d
)
For this example:
Risk of tuberculosis among east-wing residents = 28/157 = 0.178 = 17.8%
Risk of tuberculosis among west-wing residents = 4/137 = 0.029 = 2.9%
The risk ratio is simply the ratio of these two risks:
Risk ratio = 17.8/2.9 = 6.1
Thus, inmates who resided in the east wing of the dormitory were 6.1 times as likely to develop
tuberculosis as those who resided in the west wing.
■
EXAMPLE 8.21
Problem:
In an outbreak of
Varicella
(chickenpox) in Oregon in 2002,
Varicella
was diagnosed in
18 of 152 vaccinated children compared with 3 of 7 unvaccinated children. Using data contained in
Table 8.10, calculate the risk ratio.
Solution:
Risk of
Varicella
among vaccinated children = 18/152 = 0.118 = 11.8%
Risk of
Varicella
among unvaccinated children = 2/7 = 0.429 = 42.9%
Risk ratio = 0.118/0.429 = 0.28
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