its very informative

2. 2
Epidemiologic Study
Designs
Ms. Hajra Sarwar
Assistant Professor
LSN UOL
Subject Code:NUR50105

3. 3
Session Objectives
By the end of this session, the Students will be
able to:
Define the term “Epidemiology”
State the major objectives of Epidemiology
Explain the hierarchy of epidemiological designs
Identify the strengths & limitations of certain
epidemiological designs

4. 4
Epidemiology
• It is a study of distribution and
determinants of health related states or
events in a specified population and the
application of this study to control health
problems.
(Stephenson & Babiker 2010),

5. 5
Objectives of Epidemiology
 Identify the etiology or the cause of a
disease and the risk factors
 Determine the extent of disease found in the
community
 Study the natural history and prognosis of
disease

6. 6
Cont…
 Evaluate the new preventive and
therapeutic measures and new modes of
health care delivery
 Provide the foundation for developing
public policy and regulatory decisions
relating to environmental problems

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Types of Epidemiological studies
Observational
Interventional /
Experimental
-Investigator observes the
course of a disease or the
relation between the risk
factor & outcome but does
not intervene
-Investigator attempts to
prevent or change the course
of a disease.
-Are used to evaluate
preventive and therapeutic
interventions, can provide
strong evidence of causality.

8. 8
a) Descriptive studies: Examine patterns
of disease (Concerned with describing the
general characteristics of the distribution of a
disease Particularly in relation to time, place
and person)
b) Analytical studies
Studies of suspected causes of diseases
(Examines the determinants of the health
outcomes)
OBSERVATIONAL:

9. 9
Cont…
Experimental studies:
• Compare treatment modalities (RCTs)

10. 10
10
Epidemiology
studies
Observational
Descriptive
Case report
Case series
Cross sectional
Ecological
Analytical
Case control
Cohort studies
Cross sectional
Experimental

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Case Reports
 A case report is the description of a single case,
typically describing the manifestations, clinical
course and prognosis of that case and how it was
managed.
 Describe rare clinical events or unusual
manifestations of disease.
 Based on the experience of one patient only
Example:
In 1961, a published case report of a 40 year-old
women who developed pulmonary embolism
after beginning use oral contraceptive

12. 12
Case Series
• An individual case report that has been
expanded to include a number of patients with
a given disease
• Often used to delineate the clinical picture of a
disease
Example:
In Los Angeles, five young homosexuals men,
previously healthy, were diagnosed with
pneumocyst cariini pneumonia in a 6-month
period

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Strengths and limitation
Strengths
• Generate hypothesis
for further studies
• First form of
publication
• Fast and inexpensive
Limitation
• Most important
limitation: absence of a
comparison group
• No genaralizability
• Very weak to generate
causal relationships

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Cross-sectional study
• Often interest is to describe frequency and
pattern of either disease or health-related
outcome occurrence
• Usually data collected in a survey form
• Exposure and disease status are assessed
simultaneously in a well-defined population

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Prevalence
• Proportion of individuals in a population
with disease at a specific point of time
• Provides estimate of the probability or
risk at one will be ill at a point in time
• Provides an idea of how severe a problem
may be
• Useful for planning health services
(facilities, staff)

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# of existing cases of disease
P = X 100
Total population at risk
Formula for Prevalence

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Cross-Sectional Study
Example:
Hypothesis: Obesity is a risk factor for knee
osteoarthritis
Sample:
100 women living in Iqbal town Lahore

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Advantages
 Quick, easy, and cheap
 Can study multiple exposures and disease
outcomes simultaneously
 Good for describing the magnitude and
distribution of health problems

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Disadvantages
• Mixes prevalent and incident cases
• Cannot determine the temporal association

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?
“Chicken or egg” dilemma – do not know
whether the exposure preceded disease, or
was a consequence of disease development.
OR

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Ecological Study
 An ecological study focuses on groups of
people (rather than individuals) as the units of
analysis
 Uses data from entire populations to compare
disease frequencies between different groups
during the same period of time or in the same
population at different point in time

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Cont…
• The variables include measurements taken
at the group level e.g. infant mortality rates
of different countries
• A common type of ecological study design
involves comparison of treatment patterns
and outcomes of patients living in different
geographical regions or countries; such
studies attempt to make inferences about the
quality of care provided by the respective
healthcare systems.

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Yearly mortality vs. antibiotics

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Analytical Epidemiology
Analytical
Epidemiology
Analytical Cross
Sectional Studies
Case Control
Studies
Cohort Studies

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Population
Cases
Controls
Exposed
Case-Control Studies
Not
exposed
Exposed
Not
exposed

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Case-Control Study
- Aims to identify factors that may contribute to
an outcome. In this type of study, subjects are
selected based on the outcome variable; subjects
who have the condition (cases) are compared with
the subjects who do not have the condition
(controls).
• Looking back in the time, the cases and controls
are compared with regard to exposure.
(Noordzij, Dekker,Zoccali & Jager, 2009; 7 Lu, 2009)

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Strengths
•Cheap, easy and quick studies
- Multiple exposures can be examined
- Rare diseases and diseases with long latency can be
studied
-Hypothesis generating

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Limitations
Case and control selection troublesome
-Subject to bias (selection, recall,
misclassification)
-- Temporal relationship is not clear
-- some potential to generate causal relationship

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Proportions Exposed:
Cases Exposed = a / a+c Controls Exposed = b / b+d

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The Measure of Association in a Case-
control Study Is the Odds Ratio
d
c
b
a
D
E
D
E
OR = Odds of exposure among diseased
Odds of exposure among non-diseased
OR = a / c = a d
b / d b c
=

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OR > 1.0 (Positive Association)
The odds of exposure among cases is higher
than the odds of exposure among controls.
OR = 1.0 (No Association)
The cases are equally likely to have the
exposure than the controls.”
OR < 1 (Negative Association)
the odds of exposure among cases is less than
the odds of exposure among controls.
Interpretation

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Calculation of the OR in Case-control
Studies
Example: A Case-Control Study of Bladder Cancer
Cases Control
Smokers 112 176
Non-smokers
88 224
Total 200 400
thus, OR = ad/bc = (112 x 224) / (176 x 88) =1.6
Bladder cancer cases were likely to have smoked 1.6 times
more than controls.

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Population
People
without
disease
Exposed
Not
exposed
Disease
No disease
Disease
No disease
Cohort Studies

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•In a cohort study, an investigator defines
a study population consisting of subjects
who are free of the outcome of interest.
This study design aims to determine which
factors are associated with the development
of this outcome.
•Depending upon the exposure status at the
start of the study, subjects are classified as
exposed or unexposed. Thereafter the
subjects are followed over time to see who
develops the outcome and who will not.

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ineligible
no participation
deep chest SSI no deep chest SSI
participation
obese
no participation
deep chest SSI no deep chest SSI
participation
not obese
CABG surgery performed
at BJC hospitals
during 1999
CABG patients
Cohort Study

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Relative Risk
• Measure of association between incidence of
disease and factor being investigated
• Ratio of incidence rate for persons exposed
to incidence rate for those not exposed
Incidence rate among exposed
RR =
Incidence rate among unexposed
• Estimate of magnitude of association
between exposure and disease

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Incidence rate
• Incidence among exposed = a/a+b
• Incidence among non-exposed = c/c+d

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Incidence rate among exposed
RR = Incidence rate among unexposed
Formula for relative risk:
a / (a + b)
RR =
c / (c+ d)

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RR > 1.0 (Positive Association)
Risk is higher in exposed as compared to non
exposed.
RR = 1.0 (No Association)
The risk is equal in the two groups.
RR < 1 (Negative Association)
The risk in the exposed is lesser than the non
exposed.
Interpretation

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Cohort Studies
Advantages & disadvantages:
Can ascertain multiple outcome of single
exposure
High generalizability
Hypothesis generating
Can establish population-based incidence
Accurate relative risk (risk ratio) estimation

41. 41
Continue
Temporal relationship can be inferred
(prospective design)
Magnitude of a risk factor’s effect can be
quantified
Multiple outcomes can be studied
 (smoking > lung cancer, COPD, larynx
cancer)
Time consuming and costly
Loss to follow up ( participants can leave
in between)

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Epidemiologic Study Designs
Grimes & Schulz, 2002 (www)

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44. 44
Experimental Study Defined
A study in which the investigators determine
and control the conditions of the experiment:
Participants are randomly assigned to
groups that experience carefully
controlled interventions manipulated by
the experimenter allowing causal
inference about the effects of the
interventions under investigation