METHODOlOGY
EPIDEMIOLOGICAL STUDY
DESIGN
INTRODUCTION
The primary concern of epidemiologist is to study
disease occurrence in people ,who during the
course of their lives are exposed to numerous
factors and circumstances, some of which may have
a role in disease aetiology.
Unlike the clinician or the laboratory investigator,
who is able to study the disease condition more
precisely ,
the epidemiologists employs
carefully designed research strategies to
explore disease aetiology.
CLASSIFICATION
OF EPIDEMIOLOGICAL STUDIES
1- descriptive studies.
A-case reports and case series.
B-correlation studies.
C-cross sectional studies.
2-Analytic studies:
A-observational studies:
1-case control study.
2-cohort study.
B-interventional (experimental)studies:
DESCRIPTIVE EPIDEMIOLOGY
• Describes the pattern of disease occurrence in
terms of person, place, time model or host , agent
,environment model.
• Defines the relationships of disease to the
population at risk.
ADVANTAGES OF DESCRIPTIVE
STUDIES
1-They use
available data
, so there is less time,
less effort and money.
2-Describe
disease patterns
, so importance
include
-For the epidemiologists : They can identify
risk factors and formulate research questions.
-For the health administrators: identify
population at risk (whom & where) to
allocate resources and preventive programs.
Correlation or Ecological Studies
They are based on aggregate measures of exposure and
outcome from several populations.
The population is the unit of observation available for
study.
Eg: there is a positive correlation between fat consumption
and breast cancer in many nations.
Ecological studies may be more appropriate than other
designs when studying the impact of an exposure on a
community level.
Advantages
1-Quick & easy as it employs available data.
2-It is the first step in investigating a possible exposure disease relationship.
Disadvantages
1-It cannot link exposure to disease in individuals (unknown temporality)
2.correlational data represent average exposure level rather than actual
individual values.
3-It cannot control
confounders.
Case report & case series report:
case report
: The individual is the unit of observation
available for study.
Clinical case with “unusual” clinical picture
They describe the experience of a single patient or a
small number of patients with a similar diagnosis
reflecting unusual features of a disease.
They help in:
-Formulation of a hypothesis suggesting an etiological
association
-Represent the first clues in identification of new disease
or epidemic.
Case series
First case report may stimulate compilation of additional
case reports….a case series or(are collection of individual
cases report occurring within a fairly short period of time.
Advantages:
1-Use available clinical data
2-Detailed individual data
3-Suggests need for investigation (hypothesis
generation)
Disadvantages:
1-May reflect experience of one person or one clinician
2-No explicit comparison group
3-Can not be used to test the presence of a valid statistic
association.
Cross sectional (prevalence)
study
1-Collection of data on several individuals at
“one point” in time.
2-Determines prevalence at a point in time
3-Therefore, CS is a prevalence study
4-The exposure and disease status are
assessed simultaneously among individuals
in a well defined population.
5-Snapshot in time
We are interested in the possible relationship of
increased serum cholesterol level (the
exposure
) to
electrocardiographic (ECG) evidence of CHD (the
disease
). We survey a population; for each participant
we determine the serum cholesterol level and
perform an ECG for evidence of CHD. This type of
study design is called a
cross-sectional study
because
both exposure and disease outcome are determined
simultaneously
for each subject; it is as if we were
viewing a snapshot of the population at a certain
point in time.
If we determine in such a study that there appears to be an association
between increased cholesterol level and CHD, we are left with several
problems.
First
, in this cross-sectional study, we are identifying prevalent
cases of CHD rather than incident (new) cases; such prevalent cases may
not be representative of all cases of CHD that have developed in this
population. For example, identifying only prevalent cases would exclude
those who died after the disease developed but before the study was
carried out. Therefore, even if an association of exposure and disease is
observed, the association may be with
survival
after CHD rather than
with the risk of
developing
CHD.
Second
, because the presence or
absence of both exposure and disease was determined at the same time
in each subject in the study, it is often not possible to establish a temporal
relationship between the exposure and the onset of disease.
it is not possible to tell whether or not the increased cholesterol level
preceded
the development of CHD. Without information
on temporal relationships, it is conceivable that the increased
cholesterol level could have occurred as a result of the coronary
heart disease, or perhaps both may have occurred as a result of
another factor. If it turns out that the exposure did
not precede the development of the disease, the association cannot
reflect a causal relationship.
Consequently, although a cross-sectional study can be very
suggestive of a possible risk factor or risk factors for a disease,
when an association is found in such a study, given the limitations in
establishing a temporal relationship between exposure and
outcome, we rely on cohort and case- control
studies to establish etiologic relationships.
Advantages of cross sectional
study
• Provides information on frequency and
characteristics of the disease
• Standardized data collection tool.
• Able to focus data collection in specific locations or
specific groups of persons.
• May make comparisons among study participants.
• Relatively quick to do.
• May be repeated to get data on trends.
Limitations
• Inability to determine the temporal relationship
between exposure and disease.
• May be biased by lack of participation
• Reflects prevalent, not incident cases and thus
results may be related to duration of disease, or
survival with disease
DESIGN
Disease No disease
a
b
C
d
a
b
c
d
a
b
c
d
exposed
Not
exposed
Disease
No
disease
Disease
No
disease
exposed
Not
exposed
exposed
Not
exposed
To determine weather there is an association
between exposure and disease we have two
choices:
1-We can calculate the prevalence of disease in
persons with the exposure(
a/a+b
),and compare it
with the prevalence of disease in person without
the exposure(
c/c+d
).
2-or we can compare the prevalence of exposure in
persons with the disease(
a/a+c
),to the prevalence
of exposure in persons without the disease(
b/b+d
)
summary
• In cross sectional study:
• The exposure and disease status are
assessed simultaneously among individuals
in a well defined population.
• It provide information about the frequency
and characteristics of a disease by furnishing
a “snapshot”.