2. Learning Objectives:
At the end of this session, you should be able to:
– Describe the basic concepts of study designs
– Classify study designs
– Mention strengths and limitations of different study
designs
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3. Epidemiological study design
Design is arrangement of conditions for the collection,
summarization and analysis of data which enable to
answer any research question accurately and in the most
economical way
Epidemiological studies broadly categorized as
1. Descriptive studies
2. Analytic studies
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4. Introduction (…cont’d)
Purpose of Epidemiological Studies
Descriptive Analytic
Characterize disease
occurrence by time, place
and person.
Generate testable
hypothesis as to the cause of
disease
Concerned with the
search for causes and effects.
Test hypothesis about
association between
exposure and outcome.
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5. Does the study test hypothesis?
Does the study has comparison group?
Is the study unit individual?
Does the researcher intervene the
natural course of action?
Descriptive Analytical
Epidemiological study design
Correlational
/ecological/
- Case report
- Case series
- Cross-sectional
(prevalence
study)
Observational
-cohort
-Case-control
-Cross-
sectional
Interventional
/Experimental/
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No Yes
Yes
Yes
No
No
6. Descriptive study design
• Is a way of organizing data by person (who), place (where)
and time (when)
• Provides information about:
The magnitude of the problem
The populations at greatest risk of acquiring a particular
disease
The possible cause(s) of the disease
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7. Types of descriptive studies
• Correlational /ecological/
• Case report or case series
• Cross-sectional
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8. Correlational or Ecological
Uses data from entire population to compare disease
frequencies
Between different groups during the same period of
time
Or in the same population at different points in time
Does not provide individual data, rather presents average
exposure level in the community.
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9. Correlational….cont..
Cause could not be ascertained.
Correlation coefficient (r) is a measure of association
Positive association does not necessarily imply a valid
statistical association.
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10. Example
Hypertension rates and average per capita salt
consumption compared between two communities.
Average per capita fat consumption and breast cancer
rates compared between two communities.
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11. Correlational….cont..
Strengths
• Can be done
• Quickly
• An inexpensively
• Often using available data
Limitations
• Inability to link exposure with
disease
• Ecological fallacy
• Inability to control confounders
• May mask a non-linear
relationship
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12. Case Report and Case Series
Case Report
Careful and detailed report by one or more clinicians of
the profile of a single patient
Made using a hx, P/E and lab Ix
Unusual medical occurrences are documented
Provide clues in identification of a new disease or
adverse effects of exposures.
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13. Case Report and Case Series…
Case Series
Description of clinical /epidemiologic/ characteristics of
a number of patients with a given disease
It is also collection of individual case reports occurring
within a fairly short period of time.
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14. Example
The 5 young homosexual men with PCP seen
between Oct. 1980 and May 1981 in Los Angeles
created a serious concern among physicians since
PCP among young adults is not common.
Later, with further follow-up and thorough
investigation of the strange occurrence of the
cluster of cases, the diagnosis of AIDS was
established for the first time.
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15. Strengths of case report and case
series
Useful for studying signs and symptoms and
creating case definitions.
Case-series can help build up a picture of the
natural history of a disease.
Very useful in providing critical information, for
hypothesis generation
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16. Limitations of case report and case
series
Report is based on a single or few patients, which
could happen just by coincidence.
Lack of an appropriate comparison group.
Rates can not be calculated.
Detailed and complete risk factor information is
difficult to obtain for all cases from records.
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17. Cross Sectional Studies
• Assess both the exposure and the out come (disease)
statuses at the same time
• It can be useful to generate hypothesis
• By repeating at different time periods, it measures changes
• Hence evaluate interventions.
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19. Cross-Sectional Studies …
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Provide the most reliable estimate of burden of
disease
For factors that remain unaltered overtime,
such as sex, race or blood group, it can provide
evidence of a valid statistical association.
20. Cross-Sectional Studies …
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Strength
Easy to conduct and not time consuming
Can be used to compare population with different
characteristics as in comparative cross sectional studies
Limitation
• Chicken-egg dilemma
• Survivor bias
21. ANALYTIC STUDY DESIGNS
• Focuses on the determinants of a disease by testing the
hypothesis
• The ultimate goal is judging whether a particular
exposure causes or prevents disease
• Broadly classified into two - observational and
interventional studies
• Both types use "control group“ and this is the main
distinguishing feature of analytic studies
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22. Observational studies
•Information are obtained by observation of events.
• No intervention is done, or no deliberate
interference with natural course of disease.
•Cohort and case-control are the two analytical
observational study design.
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24. Definition
• Cohort
• a group of people who share a common experience or condition
• E.g. Birth cohorts, cohort of smokers, occupational exposures
• Cohort studies
• The observation of a cohort over time to measure outcome(s)
• Because the data on exposure and disease refer to different points
in time, cohort studies are longitudinal, unlike case-control studies
• Longitudinal, follow-up or incidence studies
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26. Types of cohort study
•Classified based on the temporal r/n b/n the
initiation of the study and the occurrence of the
disease
•Three terms are used to describe the timing of
events in cohort study
•Prospective
•Retrospective
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27. Prospective cohort study
Participants are grouped on the basis of past or
current exposure and are followed to the future
When the study begins, the outcomes have not yet
developed
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28. Retrospective cohort study
• Both the exposures & outcomes have already occurred when
the study begins
• Studies only prior outcomes & not future ones
• Usually less costly than prospective cohort studies and also
take less time to complete.
• Especially suitable for studies of rare exposures, or where the
latent period between exposure and disease is long.
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29. Advantages and disadvantages
Advantages of prospective cohort
• Possible to obtain more detailed information on
exposures and other key variables
• Less vulnerable to bias because the outcomes
have not occurred at the beginning
• More reliable than the retrospective if sample
size is large
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30. Disadvantages of prospective cohort
Expensive
Time consuming/results are delayed
May not be appropriate for diseases with long
induction period
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31. • Advantages of retrospective cohort
• More efficient (less time & money) for investigating
diseases that take a long time to develop (disease &
exposure have already occurred)
• Disadvantages of retrospective cohort
• Minimal information is usually available on the
exposure, outcome, and other key variables
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32. Case control study
Also known as Case-referent study
Subjects are selected on the basis of whether they do
(cases) or do not (controls) have a particular disease
under study
Groups are compared with respect to the proportion
having a history of an exposure or characteristics of
interest.
Eg. case-control study of pesticide exposure and
breast cancer
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34. Case control study is preferable to cohort study
when
1. The exposure data are difficult or expensive to
obtain
2. The disease is rare
3. The disease has a long induction/ latent period
4. Little is known about the disease
5. The population under study is dynamic
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35. Advantage of Case-Control
Relatively quick/ cheap (small “n” & no LTFU)
Can investigate a number of risk factors
Useful for rare diseases
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36. Disadvantage of Case-Control
• Vulnerable to bias
─Sampling/selection bias
─Measurements (recall bias)
• Not suitable for rare exposures
• Does not establish the sequence of events
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37. Interventional study
An investigation involving intentional change in some
aspects of the subjects.
Also called "gold standard" epidemiological study design.
E.g. introduction of a preventive or therapeutic regimen
Key Features of Experimental Design
1) Investigator manipulates the condition under study
2) Always prospective
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38. Types of interventions
1. Prophylactic - focus on prevention (e.g.
vaccines, cholesterol lowering)
2. Diagnostic - focus in evaluation of new
diagnostic procedure (e.g. comparison of a less
invasive diagnostic procedure to a gold
standard, etc.)
3. Therapeutic - focus on treatment (e.g. drug
testing, evaluation of new surgical technique,
etc.)
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39. Other ways of classifying intervention
studies
A. Classification based on the population studied
A.1 clinical trial
• Usually performed in clinical settings
• The subjects are patients
A.2 field trial
• Used in testing medicine for preventive purpose
• Subjects are healthy people e.g. Vaccine trial
A.3 community trial
• Unit of the study is group of people/community e.g.
Fluoridation of water to prevent dental caries
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40. B. Classification based on design
B.1 uncontrolled trial
• No control group
• Control will be past experience (history)
B.2 non-randomized controlled
• There is control group
• Allocation to either group is not randomized
B.3 randomized controlled
• There is control group
• There is random allocation of subjects to either group
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41. C. Classification based on objective
C.1 phase I
• Trial on small subjects to test a new drug with small
dosage to determine the toxic effect
C.2 phase II
• Trial on small group to determine the therapeutic effect
C.3 phase III
• Study on large population
• Usually randomized controlled trial
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42. Randomization
Is a process of allocation of eligible study participants
into the two group(treatment and control group) at
random.
Allocation into either group must be done after
determining eligibility and getting consent.
It provides a degree of assurance about the comparability
of the study groups that is simply not possible in any
observational design.
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43. Placebo
An inert agent indistinguishable from the active
treatment.
Use of placebo minimizes bias in the ascertainment of
both subjective disease outcomes and side effects.
Placebo effect: tendency for individuals to report
favorable response to any therapy regardless of the
physiologic efficacy of what they received.
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44. Blinding
Help in avoiding bias during treatment, monitoring
and endpoint assessment
Conditions in which blinding is not necessary:
– Outcome clearly defined and objectively measured
– If participants are less likely to be influenced by
knowledge of which intervention is given
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45. Blindness or masking
1. Non-blinded - common in community trials
2. Single blinded - the observer is aware but the subject
is not aware of treatment assignment
3. Double blinded - Neither the observer nor the subject
is aware of treatment assignment
4. Triple blinded - The observer, subject, and data
analyst are not aware of treatment assignment
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46. Challenges of Intervention Studies
Ethical considerations
Cost
Feasibility/ practical issues : Subject recruitment,
getting adequate individuals to enroll into a study is not
easy.
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