This document discusses different epidemiological research designs including cross-sectional studies, cohort studies, and case-control studies. Cross-sectional studies measure prevalence of disease or risk factors at a single point in time. Cohort studies follow groups over time to measure incidence. Case-control studies identify existing cases and look back to compare exposures between cases and controls. Each design has advantages and limitations for establishing causality and informing public health strategies.

1. UNIT 4 ( C) : EPIDEMIOLOGICAL
RESEARCH DESIGNS
Ms. Chanda Jabeen
Lecturer
RN, RM, BSN
M.Phil. Epidemiology & Public Health
PhD (Scholar) Epidemiology & Public Health
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2. LEARNING OBJECTIVES
At the end of this session, the students will be able
to:
 Understand different Epidemiological Research
Designs.
• Cross Sectional
• Case-Control
• Cohort
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3. WHY CONDUCT STUDIES?
• Todescribe burden of disease or prevalence of
risk factors, health behaviors, or other
characteristics of a population that influences
risk of disease
• Todetermine causes or risk factors for illness
• Todetermine relative effectiveness of
interventions
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4. 4

5. DESCRIPTIVE OR ANALYTIC
STUDIES
Descriptive studies
• Generate hypotheses
• Answer what, who, where, and
when
Analytic studies
• Test hypotheses
• Answer why and how
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6. CROSS-SECTIONALSTUDIES
Cross-sectional studies include surveys
People are studied at a “point” in time, without
follow-up.
Can combine a cross-sectional study with follow-up
to create a cohort study.
Can conduct repeated cross-sectional studies to
measure change in a population.
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7. EXAMPLE
According to the 2002 National Survey of Family
Growth, 35%, or about 7.4 million, of births to
U.S. women during the five years
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8. MEASURE PREVALENCE AT “POINT” IN
TIME
“Snapshot” of a population, a “still life”
Can measure attitudes, beliefs, behaviors, personal or
family history, genetic factors, existing or past health
conditions, or anything else that does not require
follow-up to assess.
The source of most of what we know about the
population
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9. DESIGN OF CROSS-SECTIONAL
STUDIES
Defined Population
Exposure +
Outcome +
Exposure +
Outcome -
Exposure -
Outcome +
Exposure -
Outcome -
Gather data on exposure
and disease
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10. TYPES OF CROSS-SECTIONAL
STUDY
Descriptive
A cross-sectional study may be purely
descriptive and used to assess the frequency and
distribution of a particular disease in a defined
population.
For example a random sample of schools across
London may be used to assess the burden or
prevalence of asthma among 12-14 year olds.
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11. ANALYTICAL
Analytical cross-sectional studies may also be
used to investigate the association between a
putative risk factor and a health outcome.
However this type of study is limited in its
ability to draw valid conclusions about any
association or possible causality because the
presence of risk factors and outcomes are
measured simultaneously.
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12. It may therefore be difficult to work out
whether the disease or the exposure came first,
so causation should always be confirmed by
more rigorous studies.
The collection of information about risk factors
is also retrospective, running the risk of recall
bias.
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13. ADVANTAGES OF CROSS
SECTIONAL STUDIES
Provides estimate of the disease burden
(prevalence)
Relatively short duration
Easy and quick
Less costly
Starting point for cohort study for screening
existing diseases
Provide wealth of data for further research
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14. DISADVANTAGES
Does not provide estimate of disease occurrence
(incidence)
No direct estimate of risk possible
Rare diseases, short duration, high case fatality not
detected
Natural history of disease information minimal
Prone for biases from selective survival
Not possible to establish temporality Therefore, it is
a weak design for proving causality
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15. COHORT STUDIES
In a cohort study, subjects with an exposure to a
causal factor are identified and the incidence of a
disease over time is compared with that of controls
(persons who do not have the exposure).
In a longitudinal study, subjects are followed over
time with continuous or repeated monitoring of risk
factors or health outcomes, or both.
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16. ADVANTAGES
Can get best assessment of exposure and can deal with
changes in exposure
May be only design if exposure needs to be measured
directly
Only way to get prospective information for rapidly
fatal diseases
Good for establishing temporal sequence and natural
history of disease
Can examine multiple outcomes linked to exposure -
often find other effects than
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17. Can estimate overall and specific disease rates,
usually incidence
Researcher selects, measures & records data
Lower potential for bias than a case-control study -
no recall and selection bias
Results are considered more conclusive than results
from case-control studies
The longer a cohort study continues, the stronger it
can become
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18. DISADVANTAGES
Costly and time consuming
Larger sample size than case-control
Data collection is usually very expensive
Long time commitment for follow-up
They are not good for diseases with a long latency
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19. COHORT STUDY DESIGN
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Study
Population
Exposure is
self-selected
Disease
Exposed
No Disease
Disease
Unexposed
No Disease
Follow
over time

20. TYPES OF COHORT
STUDIES
Prospective cohort studies
• Group participants according to past or
current exposure and follow-up into the
future to determine if outcome occurs
Retrospective cohort studies
• At the time that the study is conducted,
potential exposure and outcomes have
already occurred in the past
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21. PROSPECTIVE COHORT STUDIES
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Disease
Exposed
No Disease
Study
Population
Disease
Unexposed
No Disease
Startof
study
(Present)
(Future)

22. RETROSPECTIVE COHORT
STUDIES
Study
Population
Disease
Exposed
No Disease
Disease
Unexposed
No Disease
Start of
study
(Present)
(Past)
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23. In statistics and epidemiology, relative
risk or risk ratio (RR) is the ratio of
the probability of an event occurring (for
example, developing a disease, being injured)
in an exposed group to the probability of the
event occurring in a comparison, non-exposed
group.
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24. EXAMPLE
Risk
Disease status
Present Absent
Smoker a b
Non-smoker c d
Consider an example where the probability of developing
lung cancer among smokers was 20% and among non-
smokers 1%. This situation is expressed in the 2 × 2 table
to the right.
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25. Here, a = 20, b = 80, c = 1, and d = 99. Then the
relative risk of cancer associated with smoking
would be
RR= a/a+b
c/c+d
Smokers would be twenty times as likely as non-
smokers to develop lung cancer.
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26. INTERPRETATION:
RR > 1 Increased risk of outcome
RR = 1 No risk of outcome/No association
RR < 1 Reduced risk of outcome/Protective
Factor
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27. CASE-CONTROL STUDIES
Case-Control Studies identify existing disease/s
and look back in previous years to identify
previous exposures to causal factors.
Cases are those who have a disease.
Controls are those without a disease.
Analyses examine if exposure levels are different
between the groups.
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28. CASE-CONTROL STUDY
DESIGN
Source
Population
Cases
(Diseased)
Exposed
Unexposed
Exposed
Unexposed
Identify
cases
and
select
controls
Assess
exposure
history
Controls
(No Disease)
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29. ADVANTAGES
Best design for rare diseases
Can be accomplished quickly since events of
interest have already occurred
Can study several potential exposures at the same
time
Lends itself well to hospital-based studies and
outbreaks
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30. DISADVANTAGES
Hard to decide when disease was actually acquired
Control group can be difficult to find
Can’t calculate incidence, population relative risk or
attributable risk
HIGH potential for bias
Usually not generalizability
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31. An odds ratio (OR) is a measure of association
between an exposure and an outcome.
The OR represents the odds that an outcome will
occur given a particular exposure, compared to the
odds of the outcome occurring in the absence of
that exposure.
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32. CALCULATION
the odds of the disease in exposed over the odds
of the non-exposed
OR = odds of disease in exposed / odds of disease
in the non-exposed
= (a/c) / (b/d)
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34. INTERPRETATION:
OR > 1 Increased frequency of exposure among
cases
OR = 1 No change in frequency of exposure
OR < 1 Decreased frequency of exposure
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35. Exposure Outcome
Cohort
Case-control
Cross-sectional
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36. DIFFERENCE BETWEEN RETROSPECTIVE COHORT
STUDY & CASE-CONTROL STUDY
Case-control studies starts from outcomes to
exposure while in a retrospective cohort study one
already knows the if there was exposure and tries
identify association to outcome in a retrospective
way.
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37. ECOLOGICALSTUDY
An ecological study is an observational study
defined by the level at which data are analyzed,
namely at the population or group level, rather
than individual level.
Ecological studies are often used to measure
prevalence and incidence of disease, particularly
when disease is rare.
They are inexpensive and easy to carry out, using
routinely collected data, but they are prone to bias
and confounding
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38. WHY CARRY OUTAN
ECOLOGICALSTUDY?
An ecological study design is used when;
The purpose of the study is to monitor population
health so that public health strategies may be
developed and directed;
The purpose of the study is to make large-scale
comparisons, e.g, comparisons between countries;
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39. The purpose of the study is to study the
relationship between population-level exposure to
risk factors and disease.
Ecological fallacy, also
called ecological inference fallacy, in epidemiology,
failure in reasoning that arises when an inference is
made about an individual based on aggregate data for
a group.
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40. The Strengthening of Reporting
of Observational Studies in
Epidemiology Statement
(STROBE)
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41. STROBE provides a checklist of important
steps for conducting these types of studies, as
well as acting as best-practice reporting
guidelines.
von Elm E, Altman DG, Egger M, Pocock SJ, Gøtzsche PC, Vandenbroucke JP; STROBE
Initiative. The Strengthening the Reporting of Observational Studies in Epidemiology
(STROBE) statement: guidelines for reporting observational studies. Lancet 2007
Oct;370(9596):1453-14577. PMID: 18064739.
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42. REFERENCE
Grove, S. K., Gray, R.J.,& Burns, N.(2015).Understanding
Nursing research building an evidence- based
practice(6thed.).St. Louis: Elsevier Health Sciences.
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