The document discusses various epidemiological study designs and their classifications, including descriptive and analytical research. It outlines specific study types such as cohort, case-control, and randomized controlled trials, detailing their objectives, procedures, and advantages or disadvantages. Understanding these designs is crucial for conducting effective public health research and establishing causal relationships in epidemiology.

1. Epidemiological Methods
By: Muhammad Aurangzeb

2. Objectives
At the completion of this unit learners will be
able to:
• Define study design,
• Discuss classification of study design.
• Describe the Descriptive study designs.
• Discuss the Analytical study designs

3. Study design
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.
3

4. 4
Study design
A set of defined steps required to carry out research
on a problem under study. The design will define:
• How study subject are selected?
• Method of sample size estimation
• Procedure of data collection
• Procedure of data analysis
• Types of statistical test required

5. Cont….
• The proof for evidence-based medicine is all
collected via research, which uses a variety of
study designs.
• Different study designs provide information of
different quality.
• Therefore, you need to understand the
strengths and limitations of each type of study
design, as applied to a particular research
purpose.

6. 6
6
Classification of Epidemiological
Research/Study Designs
Descriptive
Research
Analytical
Research

7. 7
Descriptive Research
Population based
Individual based
Case reporting
Case series
Ecological /
Correlational Cross-sectional surveys

8. 8
Analytical Research
Observational Experimental /
Interventional
Randomized Control
Trials
Cohort study
Quassi
Case–control study
Cross-sectional study

9. 9
Objectives at various levels
DESCRIPTIVE STUDIES
1. Knowing the frequency of disease
2. Knowing the distribution
3. Developing the hypothesis
OBSERVATIONAL
ANALYTICAL
1. Testing the hypothesis
2. Establishing association
EXPERIMENTAL OR
INTERVENTIONAL
STUDIES
1. Strength of association
2. Establishing the cause
Study Types Objectives

10. Study Types
STUDY TYPES
Descriptive
( hypothesis formulation)
Individual based
Case studies
Case series
Population based
Ecological
Analytical/Experimental
(hypothesis testing )
Observational
Case-control
cohort
Cross-
sectional
Interventional
RCT’s (III)
Quasi-
Experimental
The researcher
studies, but does
not alter, what
occurs
The researcher
intervenes to
change reality, then
observe what
happens

11. 11
Descriptive Studies
• Describe only; do NOT examine associations
between Exposure (E) and health Outcome (O).
• Generally the purpose is to describe the variability in
a health outcome and/or formulate hypotheses.
• A descriptive study involves describing the
characteristics of a particular situation event or case.
• Descriptive studies can be carried out on a small or
larger scale.
11

12. Types of Descriptive Studies
Individual Based
Case Study
A study of one diseased individual, providing a detailed
description of an uncommon disease; provides timely
or rare information.
OR
A single patient’s clinical history is described in detail,
and then discussed in relation to the literature. Almost
always a rare unusual, or atypical case.

13. Types of Descriptive Studies
Individual based
Case Series :
A study of multiple occurrences of unusual cases that
have similar characteristics.
Investigators can calculate the frequency of symptoms
or characteristics of people with the disease.
Results may generate causal hypotheses. Neither a case
study nor a case series includes a comparison group.

14. Case Report
Case Series
One case of unusual
finding
Multiple cases of
finding
Descriptive Study Designs

15. 15
Types of Descriptive Studies
Individuals Based
Cross sectional Surveys
– Subjects or institutions are surveyed in order to
describe the prevalence of health outcomes and
/or characteristics of a population
15

16. 16
Descriptive Studies
Population Based
• Ecological
– An ecological study focuses on groups of people
(rather than individuals) as the units of analysis.
– The variables include measurements taken at the
group level e.g. infant mortality rates of different
countries.

17. 17
Types of Observational Analytical
Studies
17

18. Analytical (Non-
Intervention) Studies
Cross-
sectional
studies
Case-
control
studies
Cohort
studies

21. Cross-sectional study
A cross sectional study measures the
prevalence of health outcomes or
determinants of health, or both, in a
population at a point in time or over a short
period.

22. CROSS-SECTIONL STUDY
Information is collected from each subject at one point of time
Used to provide a snapshot of a population at a point in time
The main out-come measure is prevalence
Limited to the measurement of risk factor and out-comes at one
simultaneous point in time
Examples: screening surveys
knowledge attitude and practice (K.A.P.) surveys

23. Target Population
Sample
Gather Data on Exposure and Disease
Exposed;
Do not
have
Disease
Not
Exposed;
Have
Disease
Not Exposed;
Do not have
Disease
Begin with:
4 groups are possible
Exposed;
Have
Disease
I
Determine presence or
absence of exposure &
presence or absence of
disease

24. a b
c d
No disease
Disease
Exposed
Not
Exposed
a b
c d
a b
c d
No disease
Disease
Disease No disease
Exposed Exposed
Not
Exposed
Not
Exposed
II

25. a b
c d
a b
c d
No disease
Disease
Disease No disease
Exposed Exposed
Not
Exposed
Not
Exposed
Prevalence of disease
compared in exposed and
non exposed
a
a+b
vs.
c
c+d
Prevalence of exposure
compared in diseased and
non diseased
vs.
b
b+d
a
a+c
OR
III

27. Advantages of cross-sectional
• Outcomes and exposures measured at the same
time
• Uncovers associations for further study
• Useful for hypothesis generation
• Quick & cheap (no follow up)
• Best way to determine prevalence
• Questionnaire/interview based
• Useful for assessing practice, attitudes,
knowledge, beliefs , utilisation of services etc

28. Advantages of Cross-Sectional study
• Can be conducted to assess the health care needs of
the population
• Helpful in measuring access and utilization of health
services
• Provides information between disease and various
variables
• Provides information regarding distribution of a
disease
• Determines burden of the diseases in a population.
So helpful for planning purposes

29. Limitations of Cross-Sectional study
• No temporal or time sequence
so gives no information whether which comes first e.i. Cause or Disease
• Gives no idea about natural history of the disease or etiology
• Gives no measure of new cases occurrence
• Not useful for rare exposures or rare outcomes

31. COHORT STUDY
• Cohort studies are also called “Follow-up or Incidence
Studies”.
• Because the data on exposure and disease refer to different
points in time, cohort studies are also longitudinal.
• Cohort studies have also been called “Prospective Studies”.

32. Cohort studies
•The observation of a cohort over time to
measure outcome(s)
•Synonymous terms (Last’s)
◦Follow-up
◦Longitudinal
◦Prospective

33. Cohort studies
Birth cohort: all individuals in a defined
geographical area born in the same period (usually
a year)
Inception cohort: all individuals assembled at a
given point based on some factor e.g. Workplace

34. Cohort studies
Exposure cohort: a group of individuals that
potentially share a common exposure e.g.
Radiation
Disease cohort: a group of individuals with a
specific disease.

35. STEPS IN COHORT STUDY
• Cohort studies are conducted in three fundamental steps:
1. Identify cohorts of exposed and unexposed individuals who
are free of the disease/outcome of interest at the beginning of
the study.
2. Observe each cohort over time for the development of the
outcome(s) of interest.
3. Compare the risks of outcomes between the cohorts.

36. COHORT STUDY DESIGN

38. Prospective
Retrospective
Ambidirectional
Recruitment
Exposure Outcome
Exposure Outcome
Exposure Outcome
Exposure Outcome
Time
Cohort studies

44. COHORT STUDY DESIGN
• Cohort study measure:
i. Incidence rate
ii. Relative Risk
iii. Attributable Risk

45. DESIGN OF A COHORT STUDY
Disease
Develop
Disease
Does not
Develop
Total Incidence
Rate of
Disease
First
Select
Exposed
Not
Exposed
a
c
b
d
a + b
c + d
a/a+ b
c/c + d
Then Follow to see whether

46. INCIDENCE RATE
• Incidence in exposed group = a/ a + b
• Incidence in unexposed group = c/ c + d
• Incidence in total (exposed + unexposed)
• = a + c
a + b + c + d

47. RELATIVE RISK
• Cohort study determine whether there is an association
between exposure to a factor and development of a disease.
• Relative Risk = Incidence in exposed
Incidence in unexposed
= a/ a + b
c/ c + d

48. ATTRIBUTABLE RISK
• This is determined by the “Attributable Risk”, which is
defined as “the amount or proportion of diseases incidence
(or disease risk) that can be attributed to a specific
exposure”.
• Attributable Risk is calculated as follow:
• Risk Difference = (Incidence in exposed group ) – (Incidence
in non-exposed group [Background risk]

49. Advantages of cohort studies
• Useful for rare exposures
• Useful for more than one outcome
• Incidence of the outcome (and incidence rates)
• Temporal relationship between exposure and outcome is
clear as exposure status defined at start of study
• If prospective, minimises bias in measurement of
exposure
• Sometimes the only ethical or legal way to do study
• Stratification, nested case-controls, and multivariate
analyses (adjustment) can be applied

50. Disadvantages of cohort studies
• Not good for study of rare outcomes
• If retrospective they rely on the adequacy of
records
• Exposed may be followed more closely than
unexposed
• If prospective they can be very expensive and slow
• As they are follow up studies, the validity of results
is highly sensitive to losses to follow up (migration,
withdrawal, lack of participation, death)

51. CASE-CONTROL STUDY DESIGN

52. Case-control studies
•An analytical epidemiologic study design in which
individuals who have the disease under study, also
called cases, are compared to individuals free of
disease (controls) regarding past exposures.
•Exposure differences between cases and controls
are helpful to find potential risk or protective
factors. The purpose is to determine if there are
one or more factors associated with the disease
under study.

53. CASE-CONTROL STUDY
• To examine the possible relation of an exposure to a
certain disease, we identify;
1. A group of individuals with the disease (called cases)
and for purpose of comparison,
2. A group of people without the disease or outcome
variable (called controls ).
3. The study compares the occurrence of the possible
cause in cases and in controls.

54. DESIGN OF A CASE-CONTROL STUDY

56. Advantages of case-control Studies
• Can be carried out quickly and quite cheaply
• Useful for rare diseases and outcomes
• Can study multiple exposures for a single
outcome
• Case control studies can be ideal for the study
of rare diseases or those with a long latency
• Compares odds of exposure between cases
and controls

57. Disadvantages of case-control studies
• Selection of control population, overmatching
• Information bias as exposures – similar status
determined after outcome has occurred e.g. Recall
• Selection bias especially regarding controls
• Cannot establish sequence of events (temporal
relationship)
• Not good for rare exposures
• Cannot usually be used to estimate incidence
rates, relative risks or attributable risks

58. EXPERIMENTAL STUDY DESIGN

59. Randomised controlled trial (RCT)
”An epidemiological experiment in which subjects
in a population are randomly allocated into groups,
usually called study and control groups to receive
and not receive an experimental preventive or
therapetuic procedure, maneuver, or
interventition”

60. Randomized Control Trials (R.C.T)
Randomized:
Allocation of participants to various groups in
random fashion
Control:
A control group is used to compare the effects of a
particular treatment
Trials:
An experiment conduction.
60

61. Taraget population
Sample
Random Allocation
Intervention group Control group
Outcome measure
Changed group
during study
Loss to
follow up
Loss to
follow up
RCT
Reference population

62. RANDOMIZED CONTROLLED TRIAL
• The true experimental study design (RCT) has three
characteristics:
1. RANDOMIZATION - the researcher takes care to randomly
assign subjects to the control and experimental groups.
• (Each subject is given an equal chance of being assigned to
either group.)

63. RANDOMIZED CONTROLLED TRIAL
2. CONTROL - the researcher introduces one or more control
group(s) to compare with the experimental group.
3. MANIPULATION - the researcher does something to one
group of subjects in the study.
• Note: The strength of experimental studies is that by
randomization of confounding variables.

64. RANDOMIZED CONTROLLED TRIAL
• In Randomized Controlled Trial (RCT), we begin with a
defined population.
• Subjects in the study population are randomly allocated to
intervention and control groups, and the results are
assessed by comparing outcomes.
• The basic design of RCT is given below;

66. Allocation of study subjects -
randomization
•Random = governed by chance
•Randomization = allocation of individuals to
groups by chance
•Each sampling unit has the same chance of
selection

72. BLINDING IN RCT
• Blinding represents an important, distinct aspect of
randomized controlled trials.
• The term blinding (masking) refers to keeping trial
participants, investigators (usually healthcare providers), or
assessors (those collecting outcome data) unaware of an
assigned intervention, so that they are not influenced by that
knowledge.
• Blinding prevents bias at several stages of a trial.

75. 75
TYPES OF BLINDING
• Single Blind
– The subjects are not knowing the group to which they are
belonging .
• Double blind trials
– Neither the subject nor care giver is aware about the groups
• Triple blind trials
– The subject, the care giver (nurse or doctor) and the person
doing the analysis are not aware about the groups in.
75

76. Advantages of RCT
• Exposure in under control.
• Due to randomization both intervention and control groups
have similar characteristics.
• By blinding the study, the observer and selection bias can be
eliminated.
• If properly designed & conducted, it can reduce the
confounding.
• Can confirm or refute etiological hypothesis.
• Can evaluate the efficacy / effectiveness / efficiency of
health services.
• Best method for studying causal relationship.
76

77. Disadvantages of RCT
• Ethical problems
Due to adverse effects
Due to benefits of intervention in the treated group
Provision of Placebo
• Relatively expensive
77

78. QUASI EXPERIMENTAL STUDY
• In a Quasi Experimental Study, at least one characteristic of a
true experiment is missing, either randomization or the use of a
separate control group.
• A quasi experimental study, however, always includes
manipulation of an independent variable that serves as the
intervention.

79. QUASI EXPERIMENTAL STUDY
• One of the most common quasi experimental designs uses two
(or more) groups, one of which serves as a control group in
which no intervention takes place.
• Both groups are observed before as well as after the
intervention, to test if the intervention has made any
difference.
• The subjects in the two groups (study and control groups) have
not been randomly assigned.

81. QUASI EXPERIMENTAL STUDY
• Another type of design that is often chosen because it is quite
easy to set up uses only one group in which an intervention is
carried out.
• The situation is analyzed before and after the intervention to
test if there is any difference in the observed problem. This is
called a "Before- After" study .

83. STUDY TYPES & STRENGTH OF EVIDENCE
• Analytic Study involves the systematic evaluation of
suspected relationships, for example, between an exposure
and a health outcome.
• Analytic studies typically provide stronger evidence
concerning particular relationships.
• An experimental design is the only type of study design
that can actually prove causation.

85. References
• Principles of Epidemiology in Public Health Practice,
Third Edition An Introduction to Applied
Epidemiology and Biostatistics
• http://www.cdc.gov/
• Jhonhopkin university epidemiology lectures

86. THANK YOU
for supporting me
Best of luck in your exam