The most ambitious definition of health is that proposed by WHO in 1948: “health is a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity.” but, Practical definitions of health and disease are needed in epidemiology, which concentrates on aspects of health that are easily measurable and amenable to improvement. Definitions of health states used by epidemiologists tend to be simple.

1. Epidemiologic study
designs
1

2. 1. Introduction
Research
Research is a search for knowledge
through a careful investigation or
experimentation, aimed at the discovery
and interpretation of new knowledge.
2

3. Research
• Research is the systematic collection,
analysis and interpretation of data to
answer a certain question or solve a
problem.
3

4. Why do we need research?
Basic research is necessary to generate new
knowledge and technologies to deal with
major unresolved health problems.
Applied research is necessary to identify priority
problems and to design and evaluate policies
and programs that will deliver the greatest
health benefits, making optimal use of
available resources.
4

5. Study Design
Learning objectives
At the end of the session, trainees should be able to;
- List types of epidemiologic study designs,
- Describe advantages and disadvantages of
various study designs,
- Identify study designs on published articles,
- Outline appropriate study design for their
respective group projects,
5

6. Selection of study design
Selection of a research strategy is the core of
a research design.
Study design characteristics include:
type of data (qualitative vs quantitative),
the type of comparisons (with or without
control group),
the type of setting or unit of analysis chosen,
etc.
6

7. Selection of study design … cont’d
Selection of study design depends on:
• Type of problem
• Existing knowledge about the problem,
• Resources available for the study
7

8. Epidemiological Study Designs
Observational
• Descriptive
• Analytical
Experimental
– Randomized clinical trial (RCT)
– Community intervention trial (CIT)
8

9. Observational vs Experimental studies
Observational studies
• Investigator observes occurrence of disease
or other outcome variable, in people who
are already segregated into groups on the
basis of some exposure.
Experimental studies
• Investigator allocates the exposure.
9

10. Descriptive studies
• Characterize occurrence and distribution of
problems by time, place and person.
• Information obtained allows generation of
hypotheses, which can be tested by analytical or
experimental designs.
• Not aimed to test hypothesis
• Not structured as analytical or experimental
10

11. Descriptive studies … cont’d
• Number of events (morbidity, mortality)
enumerated,
• Population at risk identified (denominator),
• Rates, ratios, proportions calculated as
measures of probability of events,
11

12. Types of descriptive studies
• Case report
• Case series
• Descriptive cross sectional studies or
community surveys
• Ecological descriptive studies
• Trend studies
12

13. Case report
Gives a detailed report of single patient.
Classical example: In 1941 Gregs (An Australian
Ophthalmologist) reported a new syndrome of
congenital cataract linked to rubella in mother
during pregnancy.
Clinical observation such as this can give the first
clues in the identification of new disease and
effect of exposure.
13

14. Case series
Reports a series of cases of a specific condition, or a
series of treated cases. These represent the
numerator of disease occurrence, and should not
be used to estimate risks.
Example: In the 1940’s, Alton Ochenser, USA,
observed virtually all of the patients of whom he
was operating for lung cancer gave history of
cigarette smoking. Based on his case series
observation he hypothesized that cigarette smoking
is linked with lung cancer.
In classical infectious disease epidemiology, case
series often used as an early means of identifying
the presence of epidemic.
14

15. Descriptive cross-sectional studies
Cross-sectional studies entail the collection of data
on, as the term implies, a cross-section of the
population, which may comprise the whole
population or a proportion (sample) of it.
Trend studies: data may be collected at different
points in time, and changes in the pattern are
analyzed. Though different study subjects are
studied at each time, each sample can represent the
same type of population.
Ecological descriptive studies: when the unit of
observation is an aggregate (e.g. family, clan or
school) or an ecological unit (a village, town or
country) the study becomes an ecological
descriptive study.
15

16. Advantages and disadvantages of cross-
sectional studies
Advantages Disadvantages
-They are relatively quick and in
expensive.
-Often a good first step for a cohort
study.
-Provide prevalence information.
-Researcher has control over the
selection of study subjects.
-Researcher has control over the
measurements used.
-Can study several factors or
outcomes at the one time.
-Often provides early clues for
hypothesis generation.
-Does not allow the true temporal
sequence of exposure and outcome
to be ascertained, therefore unable to
shed light on cause and effect
associations.
-Potential bias in measuring
exposure.
-Potential sampling and/or survivor
bias.
-Not feasible for rare conditions.
-Does not yield incidence or true
relative risk.
16

17. Analytical studies
• Primary goal of a study is establishing a
relationship (association) between a ‘risk
factor’ (etiologic agent) and an outcome
(disease).
• Always require a comparison group.
• Basic approach is to develop a specific,
testable hypothesis, and design study to
control potential confounding,
17

18. Types of Analytical Studies
• Case-control study
• Cohort study
18

19. Case Control Studies
Case-control study design is design whereby people diagnosed as having
a disease (cases) are compared with persons who do not have the
disease (controls) to determine if the two groups differ in the
proportion of persons exposed to a specific factor or factors.
Present Past
Retrospective
study
Look for past
exposure to factor in
cases and control
Select cases and
controls
19

20. Case Control Studies (cont’d)
• Case control study design is the most commonly
used analytical strategy in epidemiology,
• It is backward-looking (retrospective) for exposure
histories of cases and controls,
• The design is more appropriate in clinical setting,
• In Case control design, comparison is made between
individuals with disease of interest and individuals
free of the disease of interest.
• Data are analyzed to determine whether exposure
was different for cases and for controls
20

21. Case Control Studies (cont’d)
Selection of cases
• What constitutes a case in the study should be
clearly defined.
• This design is particularly efficient for rare diseases
• For reasons of convenience and completeness of
case records, the cases identified for case-control
studies are often those from a hospital setting, from
physicians’ private practices, or from disease
registries.
• Newly diagnosed cases within a specific period
(incident cases) are preferred to prevalent cases.
21

22. Case Control Studies (cont’d)
Selection of controls
• It is crucial to set up one or more control groups of
people who do not have the specified disease or
condition.
• Bias in the selection of controls is often the greatest
cause for concern when analyzing data from case-
control studies.
• The sources of control groups may be:
 a probability sample of a defined population,
 a sample of patients admitted to, or attending the
same institution as the cases;
 a sample of relatives or associates of the cases
(neighborhood controls);
 a group of persons selected from the same source
population as the cases, and matched with the cases
for potentially confounding variables; 22

23. Case Control Studies (cont’d)
• The selection of controls may involve
matching on other risk factors.
• Matching means that controls are selected
such that cases and controls have the same
(or very similar) characteristics other than the
disease and the risk factor being investigated.
23

24. Case Control Studies (cont’d)
Collection of data on exposure and other factors
• Often data are collected through interviews,
questionnaires and/ or examination of records.
• The following precautions should be taken when
deciding on the data collection strategy:
Observation should be objective, or, if obtained by
survey methods, well standardized;
 the investigator or interviewer should not know
whether a subject is in the case or control group
(blinding);
 the same procedures, e.g. interview and setting,
should be used for all groups.
24

25. Case Control Studies (cont’d)
Advantages of case-control studies
• feasible when the disease being studied occurs only
rarely, e.g. cancer of a specific organ;
• relatively efficient, requiring a smaller sample than a
cohort study;
• little problem with attrition,
• sometimes they are the earliest practical
observational strategy for determining an
association.
25

26. Case Control Studies (cont’d)
Disadvantages of case control studies
• the absence of epidemiological denominators
(population at risk) makes the calculation of
incidence rates, and hence of attributable risks,
impossible;
• temporality is a serious problem in many case-
control studies where it is not possible to
determine whether the attribute led to the
disease/condition, or vice versa;
• particularly prone to bias compared with other
analytic designs, in particular selection and recall
bias,
• is inefficient for the evaluation of rare exposures.
26

27. 1. When is a case control study most
appropriate?
2. Outline the limitations of case control
study.
Exercise 1. Case control studies
27

28. Cohort studies
• Cohort study is an observational research design
which begins when group of people (a cohort)
initially free of disease (outcome of interest), are
classified according to a given exposure, and then
followed up over time.
• The researcher compares whether the subsequent
development of any new cases of a disease (or other
outcome of interest) differs between the exposed
and non-exposed groups.
28

29. Cohort studies (cont’d)
Select cohort, classify by
exposure
Follow to see frequency with
which disease develops
s
Prospective
cohort study
Schematic diagram of time factor in cohort study
Present Future
29

30. Types of cohort studies
• There are basically two types of cohort
studies;
– prospective and
– retrospective (historical).
• The difference between the two lies in
where the starting point of the study is
deemed to begin.
30

31. Retrospective Cohort
• In a retrospective cohort the ‘starting point’,
that is the point of initial exposure occurred
some time in the past and the experience of
the population is followed up to the present
time. At the time the study is initiated, both
the exposure and outcome have occurred.
• A historical cohort study depends upon the
availability of data or records that allow
reconstruction of the exposure of cohorts to a
suspected risk factor and follow-up of their
mortality or morbidity over time.
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32. Historical Cohort Studies (cont’d)
Historical cohort studies have the following
disadvantages:
 All of the relevant variables may not be available in the
original records.
 It may be difficult to ascertain that the study
population was free from the condition at the start of
the comparison. This problem does not exist if we are
concerned with deaths as indicators of disease.
 Attrition problems may be serious due to loss of
records, incomplete records, or difficulties in tracing or
locating all of the original population for further study.
 These studies require ingenuity in identifying suitable
populations and in obtaining reliable information
concerning exposure and other relevant factors.
Examples of such population groups include military
personnel, industrial groups (such as miners),
professional groups, etc. 32

33. Historical Cohort Studies (cont’d)
Data to be collected:
 data on the exposure of interest to the study
hypotheses;
 data on the outcome of interest to the study
hypotheses;
 characteristics of the cohort that might confound
the association under study.
33

34. Historical Cohort Studies (cont’d)
Methods of data collection
Several methods are used to obtain the above data,
which should be on a longitudinal basis. These
methods
include:
 interview surveys with follow-up procedures;
 medical records monitored over time;
 medical examinations and laboratory testing;
 record linkage of sets with exposure data and
sets with outcome data, e.g. work history data in
underground mines with mortality data from
national mortality files.
34

35. Cohort studies (cont’d)
Advantages of Cohort studies
• Relative risk can be calculated.
• Allows concluding a cause-effect relationship.
• No chance of bias being introduced due to awareness of
being sick as in encountered in case-control studies.
• Less chance for the problem of selective survival or
selective recall
• Cohort studies are capable of identifying other diseases
that may be related to the same risk factor.
• Allows estimating attributable risks, thus indicating the
absolute magnitude of disease attributable to the risk
factor.
• If a probability sample is taken from the reference
population, it is possible to generalize from the sample
to the reference population with a known degree of
precision. 35

36. Cohort studies (cont’d)
Disadvantages of Cohort studies
 Cohort studies are long-term and are thus not
always feasible;
 they are relatively inefficient for studying rare
conditions.
 Costly in time, personnel, space and patient
follow-up.
 Sample sizes required are large, especially for
infrequent conditions.
 Attrition or loss of people from the sample or
control during the study is the major problem.
The higher the proportion lost (say beyond 10-
15%) the more serious the potential bias.
36

37. Cohort studies (cont’d)
 There may also be attrition among investigators.
 Over a long period, many changes may occur in the
environment, among individuals or in the type of
intervention, and these may confuse the issue of
association and attributable risk.
 Over a long period, study procedures may influence the
behavior of the persons investigated in such a way that
the development of the disease may be influenced
accordingly (Hawthorne effect).
 A serious ethical problem may arise when it becomes
apparent that the exposed population is manifesting
significant disease excess before the follow-up period is
completed.
37

38. 1. What are the major difference between a
cohort and case control study?
2. What are the major limitations of a cohort
study?
Exercise 2
38

39. Experimental Studies
• The experimental study, or clinical trial, is an
epidemiologic design that can provide data of
high quality.
• Individuals are enrolled on the basis of their
exposure status,
• The distinguishing characteristic of experimental
study design is that the investigators themselves
allocate the exposure.
39

40. Experimental studies (cont’d)
• Experimental study is the best epidemiological
study design to prove causation.
• It can be viewed as the final or definitive step in
the research process.
• The experimenter (investigator) has control of the
subjects, the investigator determines who will be
exposed to the intervention and who will not.
• In Intervention studies, one or more groups with
specific interventions is compared with a group
unexposed to interventions.
40

41. Types of Experimental Studies
• Randomized clinical trial (RCT)
• Community intervention trial (CIT)
41

42. The randomized clinical trial (RCT)
• The most commonly encountered experiment in health
science research, and the research strategy by which
evidence of effectiveness is measured, is the randomized,
controlled, double blind clinical trial, commonly known as
an RCT.
• Clinical trials may be done for various purposes. Some of
the common types of clinical trial (according to purpose)
are:
a. prophylactic trials, e.g. immunization, contraception;
b. therapeutic trials, e.g. drug treatment, surgical
procedure;
c. safety trials, e.g. side effects of oral contraceptives and
injectables;
d. risk-factor trials, e.g. proving the etiology of a disease
by inducing it with the putative agent in animals, or
withdrawing the agent (e.g. smoking) through
cessation.
42

43. Randomized clinical trial (cont’d)
The intervention in a clinical trial may include:
a. drugs for prevention, treatment or palliation;
b. clinical devices, such as intrauterine devices;
c. surgical procedures, rehabilitation procedures;
d. medical counseling;
e. diet, exercise, change of other lifestyle habits;
f. hospital services, e.g. integrated versus non-integrated,
acute
versus chronic care;
g. risk factors;
h. communication approaches, e.g. face-to-face
communication versus pamphlets;
i. different categories of health personnel, e.g. doctors versus
nurses;
j. treatment regimens, e.g. once-a-day dispensation versus
three times a day.
43

44. Community intervention trials (CITs)
• The major difference between Randomized
Clinical Trials and Community intervention trials is
that the randomization is done on communities
rather than individuals.
• The classic example of a community intervention
trial would be that of testing a vaccine. Some
communities will be randomly assigned to receive
the vaccine, while other communities will either
not be vaccinated, or will be vaccinated with a
placebo.
44

45. Advantages of Experimental studies
• The ability to manipulate or assign the
exposure.
• The ability to randomize subjects to
experimental and control groups.
• The ability to control confounding and
eliminate sources of spurious association.
• The ability to ensure temporality.
• The ability to replicate findings.
45

46. Disadvantages of Experimental Studies
• Lack of reality. In most human situations, it is
impossible to randomize all risk factors except
those under examination.
• Difficulties in extrapolation.
• Ethical problems. In human experimentation,
people are either deliberately exposed to risk
factors (in etiological studies) or treatment is
deliberately withheld from cases (intervention
trials).
46

47. Disadvantages of experimental studies
(cont’d)
• Difficulties in manipulating the independent
variable.
• Non-representativeness of samples.
Many experiments are carried out on captive
populations or volunteers, who are not
necessarily representative of the population at
large.
• Experiments in hospitals (where the
experimental approach is most feasible and is
frequently used) suffer from several sources of
selection bias.
47

48. Thank
you!!!
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