Epidemiological
Studies
Epidemiological Design
Strategies:
A. Descriptive:
A.1: Population:
*Correlation studies.
A.2: Individuals:
*Case report and case series.
*Cross section
B. Analytic:
B.1: Observational studies:
*Case control.
*Cohort.
B.2: Interventional studies:
Experimental (clinical trial, lab.
Animal)
A. Descriptive studies: Describe
pattern of disease as person,
place, time.
A.1. Population:
A.1.1: Correlation studies: Describe the
disease in the entire population in
relation to factor of interest, it describe
the relation as linear association, but
sometime may be U shape or J shape.
It uses the correlation coefficient,
which is measure of association and
lies between (1-,1+) which means
strong association, but (0) means no
association.
*
Advantage:
.Quick.
. Not expensive.
. It is the first step in searching for
exposure-disease relationship.
* Limitation:
. The true in population (correlation
between disease and exposure) may
be not true on individuals.
A.2. Individuals:
A.2.1: Case report and case series: Describe
the experience of a single patient or small
group of patients with a similar diagnosis, it
reflecting unusual representation of a
disease( unusual cases e.g. polyvinyl
chloride factory that cause liver
angiosarcoma).
*Advantage:
. Formulate hypothesis.
*Limitation:
. Not population based that
means not represent
population ( no
generalization).
A.2. Individuals:
1:A.2. Cross sectional (prevalence-
transverse): Most important
The presence of disease and factor
(exposure) are assessed among
individuals in our sample at same present
time.
Advantage:
1. Measure prevalence.
2. Rapid, easy, inexpensive.
Limitation:
. Do not know which come first disease or
exposure.
B. Analytic studies:
B.1.Observational:
B.1.1. Case control (retrospective, trohoc):
Begin with group of patient (cases) and
comparable group without diseases
*Advantage:
1. Easy, not expensive.
2.Used in a rare disease.
3. Proves association.
*Limitation:
1. Selective survival.
2. Bias: recall (person not remember)
3. Difficult to select control (control must be has
the same sociodemographic and other
characteristic with the case to minimize bias)
4. Direct measures of risk is not possible, but
odds ratio is used as indirect risk measures.
Odd ratio=(a/c)/(d/b)=a/c X d/b=ad/cb
Odd ratio= Percentage of event among cases
Percentage of same event among
control group
B.1.Observational:
B.1.2.Cohort ( longitudinal, incidence):
These are observational analytic studies
where group(s) of individuals are defined
on the basis of presence or absence of
exposure to a suspected risk factor o a
disease, then followed for a period of time
to assess the occurrence of a disease.
Start with free from disease individuals.
Types of cohort:
1. Retrospective cohort: .
2. Prospective cohort.
3.Ambidirectional cohort:
Combination of both retrospective
and prospective cohort.
Cohort
RR=Ie/Io
RR=a/a+b
c/c+d
AR=Ie-Io
RR= relative risk, risk ratio.
AR= attributable risk, risk reduction.
Ie= No. of cases in exposed (a)
Total population exposed (a+b)
Io= No. of cases in non exposed (c)
Total population in non exposed (c+d)
Attributable Risk %={(Ie-Io)/Ie } X100
Cohort
Advantage:
1. Measures incidence.
2. Risk is directly measured by
relative risk and attributable risk.
3. Proves causation.
Limitation:
1. Long time and costly.
2. Not for rare disease but for rare
exposure.
3. Loss to follow up ( migration, or
death).
B.2: Interventional studies:
Like cohort studies but investigators
themselves allocate the exposure.
2:A. Lab animal: Infect animal or give a
carcinogen or new drugs.
2:B. Clinical trial: On human, either
therapeutic on a diseased people as
evaluating the effect of a certain drugs , or
preventive on a healthy people as giving a
vaccine (prophylactic).
* Advantage:
. It is a golden type of the
epidemiological studies.
*Limitation:
1.Expensive, long time.
2. Ethical problem.