codes, and even schools. For example, epidemiologists can use this type of
design to make comparisons about disease frequency or death rates for differ-
ent countries or to compare food consumption or exercise patterns in different
states. Another example is a nurse examining the relationship between child-
hood obesity and physical education classes. By plotting the average weights
and average number of hours students spent in physical education classes by
schools, a nurse could determine the association between childhood obesity
and physical education classes among schools.
Ecologic studies offer many advantages. They are expedient and relatively
inexpensive because they often rely on secondary data, which are data that have
already been collected. Ecologic studies can be used to examine a broad range
of exposures and diseases. They are useful when generating hypotheses and
may even be used to evaluate the effectiveness of population-level interventions,
such as immunizations, smoking bans, and seat belts.
Ecologic studies have disadvantages as well. The single greatest disadvantage
is that the nurse is unable to link exposure to disease with specific individuals.
For example, in the study of childhood obesity and physical education classes,
the nurse is unable to determine whether the student who participated the least
number of hours in physical education classes is obese. Another disadvantage
is when the pattern demonstrated in the population data is not the same as the
pattern shown in the individual data. To take this example further, suppose
the nurse finds an association between aggregate data of students’ weights
and aggregate data of hours spent in physical education classes in the schools
she assessed. Based on this information, the nurse might assume that obese
children spend fewer hours in physical education classes. However, if the nurse
were to look at individual student data, the nurse would find that there is no
association between weight and hours spent in physical education classes. When
an observation is made at the group level and nurses make inferences at the
individual level, it is known as the ecologic fallacy. Another disadvantage of
ecologic studies includes situations when there are differences in data collection
systems, disease definitions, treatment modalities, and survival across popula-
tion units. These differences can make comparisons difficult and sometimes
inappropriate. Also, inability to control for confounding variables and inability
to determine whether the exposure truly occurred before the disease, called
temporal ambiguity, can compromise the findings.Cross-Sectional Studies
Cross-sectional studies, also called prevalence studies, measure exposure and disease
as each exists in a defined population, or representative sample, at one specific point
in time. These studies provide a snapshot of the current health status and behaviors,
or exposures, of the population. Cross-sectional studies measure the prevalenceKEY TERMS
ecologic fallacy:
When false
assumptions
are made about
individuals based
on aggregated data
and associations
from populations
temporal
ambiguity: The
inability to control
for confounding
variables and
the inability to
determine whether
the exposure truly
occurred before the
disease
cross-sectional:
Nonexperimental
design used to
gather data from a
group of subjects
at only one point
in time; study
design used to
measure exposure
and disease as
each exists in a
population or
representative
sample at one
specific point in
time206 CHAPTER 8 Epidemiologic Designs: Using Data to Understand Populations