This document discusses various epidemiological study designs and methods. It begins by defining epidemiology and outlining key learning objectives. Descriptive studies examine disease patterns, analytical studies investigate suspected causes, and experimental studies compare treatment modalities. Observational designs include descriptive, analytical, case reports, case series, ecological studies, cohort studies, and case-control studies. Experimental designs include randomized controlled trials. The document explains each study design's advantages and disadvantages for investigating health conditions and outcomes.

1. Epidemiological Methods
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2. Learning Objectives
At the completion of this unit learners will be able to:
1. Discuss the Descriptive in term of Time, place and
Person.
2. Discuss the Analytical and basic Concepts of Cross
Sectional
3. Describe the Intervention / Experimental study
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3. Epidemiology
 Epidemiology is the study of the distribution
and determinants of health-related states or
events (including disease),
and
 the application of this study to the control of
diseases and other health problems.
(WHO 2017)
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4. Study Design /Method
A study design is a specific plan or protocol for
conducting the study, which allows the
investigator to translate the conceptual
hypothesis into an operational one.
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5. Epidemiology Methods
Experimental
(RCTs)
Observational
Analytical Descriptive
Case-Control Cohort
+ cross-sectional & ecologic
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6. Epidemiology Methods
• Descriptive studies
– Examine patterns of disease
• Analytical studies
– Studies of suspected causes of diseases
• Experimental studies
– Compare treatment modalities
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7. Epidemiology Methods
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8. Hierarchy of Epidemiologic Study Design
Tower & Spector, 2007 8
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9. Observational Studies
(no control over the circumstances)
- Descriptive: Most basic demographic studies
* Case Report
* Case Series
* Cross sectional
* Ecological/Correlation study
- Analytical: Comparative studies testing an hypothesis
* Cross-sectional
(a snapshot; no idea on cause-and-effect relationship)
* Cohort
(prospective; cause-and-effect relationship can be inferred)
* Case-control
(retrospective; cause-and-effect relationship can be inferred)
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10. Epidemiology Methods
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11. Case Report
• What?
• The profile of a single
patient is reported in
detail by one or more
clinicians
• Example
• In 1961, a published
case report of a 40
year-old women who
developed pulmonary
embolism after
beginning use oral
contraceptive
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12. Case Series
• What?
• An individual case
report that has been
expanded to include a
number of patients
with a given disease
• Example
• In Los Angeles, five
young homosexuals
men, previously
healthy, were
diagnosed with
pneumocyst cariini
pneumonia in
6-months period
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13. Ecological or Correlation
• Ecological Studies
– whole population is the unit of analysis
– relationship between exposure and outcome at the
individual level is missing (incomplete design)
– ecological fallacy
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14. Analytical Studies
(comparative studies testing an hypothesis)
• Cohort (prospective)
– Begins with an exposure (smokers and non-
smokers)
• Case-control (retrospective)
– Begins with outcome (cancer cases and healthy
controls)
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15. Population
People without
disease
Exposed
Not exposed
Disease
No disease
Disease
No disease
Cohort Studies
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16. time
Study begins here
Study
population
free of
disease
Factor
present
Factor
absent
disease
no disease
disease
no disease
present
future
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17. Advantages of Cohort Studies
 Can establish population-based incidence
 Accurate relative risk (risk ratio) estimation
 Can examine rare exposures (asbestos > lung cancer)
 Temporal relationship can be inferred (prospective design)
 Time-to-event analysis is possible
 Can be used where randomization is not possible
 Magnitude of a risk factor’s effect can be quantified
 Selection and information biases are decreased
 Multiple outcomes can be studied
smoking > lung cancer, COPD, larynx cancer)
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18. Disadvantages of Cohort Studies
 Lengthy and expensive
 May require very large samples
 Not suitable for rare diseases
 Not suitable for diseases with long-latency
 Unexpected environmental changes may influence the
association
 Non-response, migration and loss-to-follow-up biases
 Sampling, ascertainment and observer biases are still possible
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19. time
Exposure Study starts
Disease
occurrence
Prospective Cohort Study
Exposure
Study starts
Disease
occurrence
time
EPIET
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20. Retrospective cohort studies
Exposure
time
Disease
occurrence Study starts
EPIET
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21. Population
Cases
Controls
Exposed
Case-Control Studies
Not exposed
Exposed
Not exposed
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22. Case-Control Studies
Schulz & Grimes, 2002
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23. Advantages of Case-Control Studies
 Cheap, easy and quick studies
 Multiple exposures can be examined
 Rare diseases and diseases with long latency can
be studied
 Suitable when randomization is unethical
(alcohol and pregnancy outcome)
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24. Disadvantages of Case-Control Studies
 Case and control selection trouble some
 Subject to bias (selection, recall, misclassification)
 Direct incidence estimation is not possible
 Temporal relationship is not clear
 Multiple outcomes cannot be studied
 If the incidence of exposure is high, it is difficult to
show the difference between cases and controls
Reverse causation is a problem in interpretation
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25. Application Exercise:
Case / Control Study
• Describe a case/control study on the
relationship between childhood obesity,
smoking history, and occurrence of
hypertension in middle-aged men.
• What research question can we answer?
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26. Cross-Sectional Studies
• Measurement of risk and outcome at the
same time.
Risk factor
Outcome 26

27. Cross-Sectional Design
• The only study
capable of calculating
prevalence.
– Proportion of the
population with
the outcome at any
point in time.
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28. Application Exercise:
Cross-Sectional Study
• Design a cross-sectional study that examines
the relationship between dietary sodium and
hypertension in middle-aged men.
• What research question can we answer?
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29. Advantages of Cross-Sectional Studies
• Cheap and quick studies.
• Data is frequently available through current
records or statistics.
• Ideal for generating new hypothesis.
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30. Disadvantages of Cross-Sectional Studies
The importance of the relationship between the
cause and the effect cannot be determined.
• Temporal weakness:
– Cannot determine if cause preceded the effect or
the effect was responsible for the cause.
– The rules of contributory cause cannot be fulfilled.
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31. Type of
Study
Alternative
Name
Clinical Trials
Unit of
Study
Patients
Randomised Controlled
Trials
Field Trials
Community Trials
Community
Intervention Studies
Healthy People
Communities
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32. Types of trials
Blinded Not blinded
Randomised Not randomised
Controlled Not controlled
Trial
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33. time
Study begins here (baseline point)
Study
population
outcome
Intervention
no outcome
outcome
Control
no outcome
baseline
future
RANDOMIZATION
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34. Intervention Randomisation
Control Blinding
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35. Experimental Studies are Useful for Evaluating
• New drug or other treatment for disease
• New medical/health care technology
• Methods of prevention
• Methods of health promotion
• New health protection policies
• Programs for screening and diagnosis
• Methods of providing health care
• New health care policies
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36. Ethical Considerations in
Experimental Studies
• Is proposed treatment safe?
• For the sake of trial, can a treatment ethically be
withheld?
• What patients may be brought into trial and
allocated randomly to treatments?
• Is it ethical to use a placebo or dummy treatment?
• Is it proper for the trial to be in any way masked?
Adapted from Hill (1977)
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37. Advantages (I)
– the “gold standard” of research designs.
They thus provide the most convincing
evidence of relationship between exposure
and effect. Example:
• trials of hormone replacement therapy in
menopausal women found no protection
for heart disease, contradicting findings
of prior observational studies
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38. Advantages (II)
 Best evidence study design
 No selection bias (using blinding)
 Controlling for possible confounders
 Comparable Groups (using randomization)
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39. Disadvantages
• Is the most expensive study design in terms of
money, time, and number of patients.
– Issues of patient attrition and compliance may
invalidate the results.
– Can be problematic for ethical reasons.
• Use of placebo
• Harm outweighing benefits
• Zero tolerance for some exposures
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