Environmental Engineering Reference
In-Depth Information
is of interest to determine the incidence of disease in the exposed group vs. the
unexposed group. To do this, a 2
2 table is constructed as previously illus-
trated and a relative risk (RR) is determined. Relative risk cannot be established
for a case-control study because members of the case-control population are not
random samples of the
entire
community population.
To illustrate the calculation of RR, a hypothetical situation is presented below.
The same data as for the case-control study was used for comparative purposes:
×
52 people drank contaminated water and became clinically ill. (
a
)
32 people drank contaminated water and did not become ill. (
b
)
21 people did not drink contaminated water and became ill. (
c
)
64 people did not drink contaminated water and did not become ill. (
d
)
The 2
×
2 table is constructed to display the data.
Did not
Drank water
drink water
Became ill
52
21
Did not become ill
32
64
Calculation of the RR value involves the ratio of the exposed group as a
proportion of the population examined to the unexposed group as a proportion
of the population examined:
a/(a
+
b)
c/(c
+
d)
=
52
/(
52
+
32
)
21
/(
21
0
.
62
0
.
24
=
2
.
6
RR
=
64
)
=
+
The RR establishes that the relative risk of becoming ill for the group of
people exposed to contaminated water as opposed to the group of people not
exposed to contaminated water is 2.6.
Two types of information regarding disease in a population that can be helpful
to an epidemiological study are incidence rate and prevalence rate. Incidence rate
is defined as the number of new cases per unit of person-time at risk. For example,
suppose the waterborne outbreak used in the previous examples occurred in a
stable community of 10,000 people. Following the outbreak, the number of new
cases occurring over a five-year period was 30 per 10,000 people. These new
cases might have nothing to do with consuming water, but the waterborne incident
might have established some carriers of the disease within the population that
could contribute to the infection of others. In this example, the incidence rate
of the disease in the community would be 6 cases per 10,000 people-years; the
expression
people-years
arriving from the normalization of the 30 disease cases
over a five-year period.
Prevalence rate is something different from incidence rate because prevalence
rate concerns the actual number of disease cases in a community. In the case