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1. Research methods and materials
Gizachew A (MPHE)

2. learning objectives
• At the end of the session students are expected to be
acquainted with:
 The classification of the basic epidemiological designs,
 When to use a specific study design,
 The strengths and weaknesses of the study designs,
 How to operetionalize the specific study designs,
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3. Methods and Materials
The methods or procedures section is really the heart
of the research proposal
Methods/procedures show how you will
achieve the objectives and
answer the research question
Indicates the methodological steps you will take
to answer every question,
to test every hypothesis or to address every objective
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4. Methods section includes:
 Study design,Study area and period
 Populations
 Study variables
 Eligibility (Inclusion and exclusion) criteria
 Sample size calculation, sampling methods & procedures
 Data collection techniques & tools
 Data quality control measures
 Plan for data processing and analysis
 Operational definitions
 Ethical considerations
 Plan for dissemination and utilization of findings
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5. Study Area and Period
• Study Area
 Brief description of:
– the area,
–the population,
–the health service,
–study subjects distribution etc,
 (from national census or other research
descriptions).
 Study priod(data collection period)
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6. 2/22/2023 6
Study design
• The selection of study design depends on:
• The type of problem
• The current knowledge about the problem
• Availability of resources
• Different research questions may require
different study designs
• The selection of an appropriate study design for the
study is the most important decision the investigator
has to make.

7. Introduction to study designs
• Is a specific plan/protocol for conducting a study, which
allows the investigator to translate the conceptual
hypothesis into an operational one.
• Or it is the study approach to answer the research
question.
• The basic epidemiologic research designs are
categorized into:
– Descriptive
– Analytic
• Descriptive study is limited to a description to the
occurrence of a disease in a population by place, person
and time.
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8. Introduction cont...
• Analytic study goes further by analyzing relation between the health
outcome with possible determinants.
• Analytic studies are further classified into:
– Observational
– Interventional (Experimental).
• Observational studies allow nature to take its course: i.e. the investigator
measuresbut does not intervene.
• But interventional studies involve an active attempt by the investigator
to change a disease determinant or the progress of adisease through
treatment.
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9. 9
Introduction (cont.)
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10. Ecological/correlational study
• Is an epidemiological study in which the unit of
analysis is a population rather than an individual.
• Commonly Correlation Coefficient (r) is used as a
measure of association betw een disease and exposure.
• The populations compared in various ways.
– Geographical comparison: Look for geographical
correlations b/n disease incidence or mortality &
prevalence of risk factors.
– Time trend comparison: Look for the association b/n
exposure and health outcome across different points in
time.
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11. Ecological study
Advantagesof ecological studies:
• Often the information about disease and exposure is
abstracted from published statistics and therefore
does not require expensive or time consuming data
collection.
• It helps to generate a hypothesis.
• Used to examine rates of disease in relation to a
population‐level factors
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12. Ecological study
Disadvantagesof ecological studies:
• Exposure and disease are not linked at individual level.
• Lack of ability to control for effect of potential confounders.
• The ecological fallacy: The average value of the exposure and
outcome may not apply to all the individualsin a population.
• As a result an association observed b/n variables on an aggregate
level does not necessarily represent the association that existsat
individual level.
• Diagnostic criteria used in different population and time migh be
different.
• Difficulty to detect non‐linear association between exposur and
outcome
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13. Ecological study
Example of ecological studies with prominent findings:
– Association b/n male circumcision and decreased risk
of HIV infection is noted through ecological studies in
late 1980’s.
– Association b/n the sale of inhaled bronco‐dilators
without prescription with asthma death in UK in 1950’s
and 60’s.
– Association b/n prevalence of cigarette consumption
and death rate from cancer of the lung 1900‐1
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14. Case Report/Series
• This is a descriptive study of a single individual (case
report) or small group (case series)
• It usually describes the clinical presentation of a rare
disease based on detailed clinical evaluation and histories
• It can also discussthe possibility of an association b/n an
observed effect and a specific exposure.
• Case report/seriesare usually considered:
– An unexpected association between exposure and disease,
– An unexpected association between diseases,
– An unexpected event in the course of observing or treating a
patient.
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15. Case Report/Series
Advantagesof case report/series:
– Usually provides the first evidence of innovative treatment and
emergence of a disease.
– Useful when the disease is uncommon.
– Useful to generate hypothesis
Disadvantagesof case report/series:
– Report is based on single or few patients
– No control group is involved/comparision
– Lack of a denominator to calculate rates of disease
Example:- One of the major findings based on case report is
identification of AIDS among 5 homosexual men with PCP
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16. Cross Sectional Studies
• Also know by the name survey or prevalence studies.
• It is the most commonly used epidemiologic study design.
• The typical feature of cross‐sectional study is it involves
single period observation.
• Such studies are usually descriptive.
• Thus they focus on the assessment of the level exposure or
outcome at point in time.
• It is also possible to examine the relationship b/n a disease
and an exposure among individualsin a defined population at a
point in time.
• The later is sometimes called cross‐sectional comparative
study or cross‐sectional study with internal comparison
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17. Cross Sectional Studies
Advantagesof crosssectional studies:
– Useful for hypothesis generation.
– It is less resource and time consuming.
– provide generalizeable findings.
– Repeated crosssectional studies can indicate trend in disease or risk factor
Disadvantagesof crosssectional studies:
– Usually affected by “chicken or egg”dilemma.
– Not suitable for study of rare situations.
– Not suitable for the study of acute disease.
– Liable to “survivor bias” (obtaining data only from those who
have survived to provide it)
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18. Case-control studies
• Subjects are selected with respect to presence or absence
of the outcome of interest (e.g. disease), and then inquiries
are made about past exposure to the factors of interest.
• Those who have the outcome of interest are termed
as‘cases’, and those who do not have the outcome of
interest are termed as ‘controls’.
• The exposure histories of cases and controls are then
obtained and compared.
• Thus, the central feature of case control study is the
comparison of the cases’ and controls’ exposure histories.
• Case control study is always retrospective in nature
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19. Case-control….cont
• G
Study pop
cases
controls
exposed
Not exposed
Not exposed
exposed
Study begins here
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20. Case-control….cont
Steps:
Define Research Question
Identify Case Population
Identify Control Population
Measure Variable
Compare the exposure variable in both cases and controls
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21. Case-control….cont
Selection of cases:
A/ Definition- precise definition of cases(clinical
,laboratory and other criteria)
B/ Inclusion and exclusion criteria
-cases should be selected to improve validity(ex. By
excluding cases with coexisting disease)
-cases should be restricted to limited time period and
geographic area, age range etc.
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22. Case-control….cont
C/ Incident or prevalent cases?
To the extent possible avoid prevalent cases. Why?
• Chicken-egg dilemma on cause and effect
• Non-representative cases since long-time survivors are
selected
• Prevalent cases may not accurately recall antecedent events
• Difficult to distinguish prognostic factor and cause
NB. Prevalent cases are commonly used in studies of chronic
conditions with ill-defined onset times
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23. Case-control….cont
Sources of Cases:
– Hospitals, other medical care facilities, etc.
– General population - locate and obtain data from all or
a random sample of individuals from a defined
population
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24. Case-control….cont
Selection of controls:
Controls are free of the diseases that should represent the
population at risk of becoming cases (controls & cases
should came from the same source population)
The prevalence of exposure among controls should reflect
the prevalence of exposure in the source
population/repesentative .
The time during which a subject is eligible to be a control
should be the time in which the individual is also eligible to
be a case.
• If the above three points are not fulfilled=selection bias
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25. Case-control….cont
Sources of Controls:
• General population
• Neighborhood
• Friends/relatives
• Hospital or clinic-based
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26. Case-control….cont
• For each control group, how many controls per case?
• the optimal case-control ratio is 1:1
• when the number of cases is small, the sample size for the
study can be increased by using more than one
control
– e.g. 1:2 1:3 1:4
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27. Case-control….cont
Ascertaining Exposure:
Sources of exposure data (cases and controls):
---Study subjects (self-report). Particularly vulnerable to
recall bias as cases may recall their exposure history more
thoroughly than controls.
-
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28. Case-control….cont
Strengths:
 Is relatively quick and inexpensive
 Is optimal for the evaluation of rare diseases.
 Can examine multiple etiologic factors for a single disease.
Limitations:
 Not efficient for studying rare exposures
 Subject to biases (recall & selection bias)
 Cannot directly compute incidence rates of disease
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29. Case-control….cont
Analysis: comparison is made by calculating OR
OR
=
odds of disease in exposed
odds of disease in unexposed
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30. 400
200
Total
224
88
Non-
smoker
176
112
Smoker
Exposure
Status
No CHD
(Controls)
CHD cases
(Cases)
Disease Status
Calculating the Odds Ratio
Odds Ratio = =
AD
BC
112 x 224
176 x 88
= 1.62
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31. Cohort studies
 The term ‘cohort’ means group of people with a common
characteristicor experience.
 Cohort study is a design in which two groups are defined
according to their exposure status (exposed and unexposed
groups) and the rate of occurrence of the interest of outcome is
compared between the groups to explore association between
the exposure and outcome.
 Cohort study is also known as, longitudinal, follow up study or
incident study
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32. Purposes of cohort study
They have 2 primary purposes:
• Descriptive (measures of frequency):describes the
incidence rates of an outcome over time, or simply
describes the natural history of disease
• Analytic (measures of association):analyzes associations
between the rates of the outcomes and risk factors
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33. Types of cohort study
• Depending on temporal relationship between
initiation of the study /timing of data collection/ and
occurrence of the disease , cohort studies
prospective and retrospective cohort studies
• Prospective and
• Retrospective
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34. Prospective cohort studies
• Exposure and outcome data is collected after start
of the study
– cohorts identified in the present
– exposure status or possible explanatory/prognostic factors
determined in the present
– Cohorts followed-up to identify outcome
– Ascertainment of outcome done in future
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35. 2019 2018
2022
Fig. 2. Design of prospective cohort studies
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36. Retrospective cohort (1)
• All the exposure and effect have occurred before
the actual study begins
• This type of investigation is called a historical
cohort study
• Conduct
– Identify cohort in the past using records/databases
– Determine exposure or prognostic factors in the past using
again records or databases and identify outcome in past
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37. Retrospective cohort (2)
• Costs can occasionally be reduced by using a historical
cohort (identified on the basis of records of previous
exposure)
• This sort of design is relatively common for studies of
cancer related to occupational exposures
• For example, records of military personnel exposure to
radioactive fall-out at nuclear bomb testing sites have
been used to examine the possible causal role of fall-out
in the development of cancer over the past 30 years
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38. Retrospective vs. Prospective
• Can be conducted more quickly and cheaply
– All relevant events have occurred
– Efficient for disease with long latency periods
• Depend on availability of routine data
– Incompleteness
– Lack of data on confounding variables
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39. 2018 2019
2020
Fig. 3. Design of retrospective cohort studies
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40. Closed and Open cohorts:
• There are two types of cohortst hat epidemiologists follow :
Closed and Open cohorts
• A closed cohort is one with a fixed membership.
• Once it is defined and follow‐up begins, no one can be added
to a closed cohort.
• However, as people in the cohort die, are lost to follow‐up, or
develop the disease they are excluded from follow‐up.
• In contrast, an open cohort, which is also referred to as a
dynamic cohort, can take on new members as time passes.
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41. Analysis
• RR=INCIDENCE EXP/INCIDENCE NONEXP
• Presence of association
– Population
• RR=1 – no association; RR<1 – negative association;
RR>1 – positive association
• AR, PAR &PAR %
– Sample
• P-value<0.05 – statistically significant association
• RR≠1 – statistically significant association
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42. Disadvantages (1)
• Time-consuming - prospective
– Require long periods of follow-up since disease may occur a
long time after exposure
– For example, the induction period for leukaemia or thyroid
cancer caused by radiation is many years and it is necessary
to follow up study participants for a long time
– Many exposures investigated are long-term in nature and
accurate information about them requires data collection over
long periods
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43. Disadvantages (2)
• Expensive - prospective
– Often involve following large number of individuals for
many years
– Exposure assessment
• Incompleteness of records – retrospective
• Loss-to-follow-up – bias (not tolerable if >15%)
• Inefficient for rare disease
• example : Cohort study on British doctors to assess the
association between smoking and lung cancer (1954‐1980’s)
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44. Experimental study
• An experimental study, also know n as a trial, investigatesthe role of
some agent in the prevention or treatment of a disease.
• The investigator assigns individuals to two or more groups that
either receive or don’t receive the preventive /therapeutic agent.
• The group that is allocated the agent under study is generally called
the treatment group, and the group that is not allocated the agent
under study is called the comparison group.
• Depending on the purpose of the trial, the comparison group may
receive:
– No treatment at all,
– An inactive treatment such as a placebo, or
– Another active treatment (positive control)
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45. Experimental study
• The active manipulation of the agent (exposure) by the
investigator is the hallmark that distinguishes experimental
studies from observational ones.
• In the latter, the investigator acts as apassive observer merely
letting nature take its course
• An experimental design is a study design that gives the most
reliable proof for causation, because of individuals are
randomly allocated to each groups .
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46. Experimental study….
types Experimental study based population involved
• Clinical Trials:usually performed in clinical setting and
the subjects are patients.
• Field Trials:used in testing medicine for preventive
purpose and the subjects are healthy people.
• Community Intervention Trials:a field trial in which
the unit of the study is group of people/community.
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47. Based on Design:
– Uncontrolled trial: Involves no control group.
– Non‐randomized control: There is control
group but allocation into either group is not
random.
– Randomized control: There is control group
and allocation into either group is randomized.
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48. Population
Patients
with a
disease
Treatment
Placebo
Recover
Recover
Not
recovering
Not
recovering
Direction of inquiry
Time
Experimental study design (Clinical trial)
Manipulation
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49. Population
People
with out a
disease
Intervention
No-intervention
Disease
Disease
No disease
No disease
Direction of inquiry
Time
Experimental study design (field trial)
Manipulation
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50. Population
Community
with out a
disease
Intervention
No-intervention
Disease
Disease
No disease
No disease
Direction of inquiry
Time
Experimental study design (community intervention)
Manipulation
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51. Conducting Clinical/Field Trials
Essential steps:
• Formulate a hypothesis
• Select study participants and measure base line
characteristics
• Choose the treatment arms
• Allocate subjects by randomisation
• Apply intervention
• Follow up study subjects and collect outcome data
• Analysis(usually the main measure of effect is relative risk
or risk ratio)
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52. Reference
Population
Study
Population
R
Treatment arms
Follow up
Outcomes
Conducting Clinical/Field Trials….
• I
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53. 53
Community Trials
• Are experimental trials where the units of
analysis are entire communities or groups of
people
– E.g., Cities, towns, villages, schools, occupational
settings
• major objectives:
Evaluate the impact of health-related programs, projects, or
campaigns on communities (EVALUATION STUDIES)

54. 54
Examples of community trial interventions
• Educational interventions
• Water fluoridation to prevent dental carries
• Nutritional, environmental sanitation interventions
delivered to household, village etc
– latrines, dietary supplements

55. 2. Quasi-experimental studies
• Are trials in which either randomization or control
group is missing.
• Usually no randomization-two comparison groups in
which one serves as control
– E.g. – effect of health education on immunization

56. Identify the most appropriate study design for
the research proposal you are planning to
develop.
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57. .
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