epidemiological

1. Epidemiological Study Designs
By Addisu W. (MPH, Epi&Bio)

2. Session Objectives
By the end of this session, students should be able to
• Distinguish between interventional and observational studies.
• Describe the key characteristics of experimental, cohort, case—
control, cross-sectional, and ecologic studies regarding subject
selection, data collection, and analysis.
• Identify the design of a particular study.
• Discuss the factors that determine when a particular design is
indicated.
• Identify the strength and limitation of each studies
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3. Overview
 What is design???
 Arrangement of conditions for the collection & analysis
of data
 Logical model that guides the investigator in the
various stages of the research process
 Overall structure of the study
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4. Overview…
• There are several types of study designs.
• The selection of one design over another
depends on the particular
– Research question,
– concerns about validity and efficiency,
– practical and ethical considerations.

5. Overview…
• Generally study design divided into two
– Observation
– Experimental
• Observation study designs
– Descriptive( ecological, case series, case report,
cross-sectional)
– Analytical(cross-sectional, case-control, cohort

6. Observational vs. Experimental
• Experimental
– The investigator assigns individuals to two or more groups that
either receive or do not receive the preventive or therapeutic
agent
• study of the effect of drinking alcohol on the developing
foetus by assigning newly pregnant women to either a
drinking or nondrinking group. ????
• Observational
– The investigator lets nature take its course
• comparing women who choose to drink during pregnancy
with those who decide not

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8. Selection of Study Design
• Selection of a design depends on:
1) State of knowledge on the problem
2) Research questions
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9. Selection of study design…
• State of knowledge of the problem
– Knowledge that problem exists, but knowing little about its
characteristics of possible causes
• Types of research questions
– Who is affected?
– How do the affected people behave?
– what do they know, believe, think about the problem?
– What is the magnitude of the problem?
• Study design
– Qual. (e.g. FGD) Or Quant. (Descriptive.)
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10. Selection of study design…
• State of knowledge of the problem
– Suspecting that certain factors contribute to the problem
• Types of research questions
– Are certain factors indeed associated with the problem?
• Study design
– observational analytic
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11. Selection of study design…
• State of knowledge of the problem
– Having sufficient knowledge about the cause
– To develop & assess an intervention that would prevent,
control, or solve the problem
• Types of research questions
– What is the effect of a particular intervention?
– Which of the alternative strategies gives better result?
• Study design
– Intervention
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12. Descriptive Study Designs

13. 1. Case report and case series
• Case reports describe the experience of a single
patient
• Case series describe the experience of a group
of patients with similar diagnosis
• Both document unusual occurrences
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14. Example of case series
 In 1974, Creech and Johnson reported a case series of three
men with angiosarcoma of the liver among workers at a vinyl
chloride plant. This number in such a small population during the
time period studied was clearly in excess of what was expected.
This led to the formulation of the hypothesis that occupational
exposure to vinyl chloride caused hepatic angiosarcoma. Later
the same year, this hypothesis was substantiated/approved by
data from two analytic studies.
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15. Limitations of case report/series
• The presence of any risk factor may be
coincidental.
• Interpretation of information from case series is
severely limited by the lack of an appropriate
comparison group.
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16. 2. Cross sectional studies (surveys)
• Examines the relationship between diseases (or
other health related characteristics) and other
variables of interest at one particular time.
• Populations are commonly selected without
regard to exposure or disease status.
• Cross sectional studies are carried out for:
 public health planning
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17. Cross sectional studies (surveys)…
• subjects are sampled without respect to
disease/exposure status and are studied at a
particular point in time.
• The term "cross-sectional study" (or "prevalence
study") usually refers to studies at the individual
level.
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18. Cross sectional studies (surveys)…
• conducted to estimate prevalence is called a
prevalence study
• Need to be conducted using representative
sample
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19. Cross sectional studies (surveys)…
• Usually current disease status is examined
in relation to current exposure level.
• possible to examine disease prevalence in
relation to past exposure if the dates of the
exposure are ascertained.
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20. Advantages of cross sectional studies:
• One-stop, one-time collection of data
• Less expensive & easier to conduct
• provide much information useful for planning health
services and medical programs
• show relative distribution of conditions, disease,
injury and disability in groups and populations
• studies are based on a sample
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21. Disadvantages
1. chicken or egg dilemma -which occurred first? the
exposure or the outcome ?
– This occurs when exposure under study is changeable
characteristic such as place of residence, habit such as
cigarette smoking
• Eg. Cigarette smoking and the risk of tubal
infertility
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22. Disadvantages…
• Result: the frequency of cigarette smoking is three
times greater among the infertile women.
• But: one could conclude that there is a moderately
strong association between smoking and tubal
infertility.
• is smoking caused the infertility???
– women may have begun smoking after difficulties of
achieving a pregnancy.
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23. Disadvantages…
• The temporal inference problem can be avoided if an
inalterable characteristic is the focus of the
investigation. E.g. genetic trait
• It can also be avoided if the exposure measure
reflects not only present but also past exposure.
– E.g. an X-ray fluoroscope (XRF) measurement of an
individual’s bone lead level reflects that person’s cumulative
exposure over many years.
• Study of infertility & bone lead levels using XRF measurements would
not suffer from temporal relationship problem.
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24. Disadvantages…
2) Length-biased sampling – the cases identified
will over-represent cases with long duration
(prevalent cases) and underrepresented those
with short duration of illness.
People who die soon after diagnosis or who recover
quickly are less likely to be identified as diseased.
This can bias the results if duration of disease is
associated with the exposure under study.
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25. Disadvantages…
3) “Healthy worker survivor effect,” In studies
conducted in occupational settings, because these
studies include only current and not former
workers, the results may be influenced by the
selective departure of sick individuals from the
workforce
– Those who remain employed tend to be healthier than those who
leave employment.
– This phenomenon known as the “healthy worker survivor
effect,” generally attenuates an adverse effect of an exposure
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26. Analysis
 Either compare prevalence rate of the outcome in exposed Vs non-
exposed, or
 Compare prevalence rate of the exposure in those with and without the
outcome
 Timing of the subdivision of the study population into comparison
groups distinguishes cross sectional studies from other observational
analytic studies
 In cohort and case control studies, this takes place prior to the data
collection process
 In a cross sectional study, this takes place after the information has
been collected
 For analytical crossectional study OR is the measure of
association 26

27. Correlation (Ecological) Studies
 A classic ecologic study examines the rates of disease
in relation to a factor described on a population level
 The units of analysis are populations or groups of
people rather than individuals
 The population level factor may be an aggregate
measure that summarizes the individual members of
the population
 Ecologic studies usually identify groups by place,
time, or a combination of the two
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28. Correlation (Ecological) Studies…
The two key features that distinguish a
traditional ecologic study from other types
of epidemiologic studies are:
1. The population unit of analysis
2. An exposure status that is the
property of the population
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29. Correlation (Ecological) Studies…
Example 1
 to describe patterns of mortality from CHD in 1960, death rates
from 44 states were correlated with percapita cigarette sales in
USA.
 Death rates were highest in states with the most cigarette sales
 This contributed to the formulation of the hypothesis that cigarette
smoking causes fatal CHD
 This has been substantiated in a large number of subsequent
analytic epidemiologic studies.
 The correlation coefficient (r) is the descriptive measure of
association.
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30. Correlation (Ecological) Studies…
Age-adjusted annual coronary heart disease incidence per 100,000
35 Finland
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25 Sweden
Norway R
2
= 0.53
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UK US Denmark
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Netherlands
10 NZ Canada Austria
5 Australia France
Japan
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Average annual coffee consumption (kg/person)
1978-1982
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31. Correlation (Ecological) Studies…
Strength
 can be done quickly and inexpensively, often
using already available information
Limitation
1) Since the unit of analysis is a population or
group, the individual link between exposure and
effect cannot be made
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32. Strengths and Limitations cont.…
• The lack of individual-level information leads to
“Ecological fallacy” or “ecological bias”
 an association observed between the variables on an
aggregate level does not necessarily represent the
association that exists at the individual level.
2) lack of ability to control for the effects of
potential confounding factors
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33. Thank you!!!
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