Geoscience Reference
In-Depth Information
8.6 MEASURES OF ASSOCIATION
The key to environmental health analysis is comparison. Occasionally we might observe an inci-
dence rate among a population that seems high and wonder whether it is actually higher than what
should be expected based on, say, the incidence rates in other communities. Or, we might observe
that, among a group of case-patients in an outbreak, several report having eaten at a particular res-
taurant. Is the restaurant just a popular one, or have more case-patients eaten there than would be
expected? The way to address that concern is by comparing the observed group with another group
that represents the expected level.
A measure of association quantifies the relationship between exposure and disease among the
two groups. Exposure is used loosely to mean not only exposure to foods, mosquitoes, a partner
with a sexually transmissible disease, or a toxic waste dump, but also inherent characteristics of
persons (for, example, age, race, sex), biological characteristics (immune status), acquired charac-
teristics (marital status), activities (occupation, leisure activities), or conditions under which they
live (socioeconomic status or access to medical care).
The measures of association described in the following section compare disease occurrence
among one group with disease occurrence in another group. Examples of measure of association
include risk ratio (relative risk), ratio, odds ratio, and proportionate mortality ratio.
8.6.1 r
isK
r
atio
A risk ratio (RR), also called
relative risk
, compares the risk of a health event (disease, injury, risk
factor, or death) among one group with the risk among another group. It does so by dividing the
risk (incidence proportion, attack rate) in group 1 by the risk (incidence proportion, attack rate) in
group 2. The two groups are typically differentiated by such demographic factors as sex (e.g., males
vs. females) or by exposure to a suspected risk factor (e.g., did or did not eat potato salad). Often,
the group of primary interest is labeled the exposed group, and the comparison group is labeled the
unexposed group.
8.6.1.1 Method for Calculating Risk Ratio
The formula for risk ratio (RR) is
Risk of disease(incidenceproportion, attack rate)ingroup of primaryinterest
Risk of disease(incidenceproportion, attack rate) in comparison group
A risk ratio of 1.0 indicates identical risk among the two groups. A risk ratio greater than 1.0 indi-
cates an increased risk for the group in the numerator, usually the exposed group. A risk ratio less
than 1.0 indicates a decreased risk for the exposed group, indicating that perhaps exposure actually
protects against disease occurrence.
■
EXAMPLE 8.20
Problem:
In an outbreak of tuberculosis among prison inmates in South Carolina in 1999, 28 of 157
inmates residing on the east wing of the dormitory developed tuberculosis, compared with 4 of 137
inmates residing on the west wing (McLaughlin et al., 2003). These data are summarized in the
two-by-two (2×2) table (see Table 8.9A), so called because it has two rows for the exposure and two
columns for the outcome. In this example, the exposure is the dormitory wing and the outcome is
tuberculosis (see Table 8.9B). Calculate the risk ratio.
Solution:
To calculate the risk ratio, first calculate the risk or attack rate for each group. Here are
the formulas:
Search WWH ::
Custom Search