epidemiology note '' Epidemiological study design ''

1. CHAPTER FOUR
EPIDEMIOLOGICAL STUDY DESIGN

2. What is research?
–is the systematic collection, analysis &
interpretation of data to answer a certain
question or solve a problem.
–Health Research, is a research that address
health related problems & come up with
better solutions to mitigate the prevailing
problem

3. Why Epidemiological Studies?
To answer questions like:
– How big is the problem (magnitude)?
• Prevalence, incidence mortality
– Who has the problem?
• Person characteristic of affected population
– Where is the problem?
• Place characteristics (locality)
– When was the problem
• Time characteristics
– What causes the problem
• Other specific factors related to causation

4. Classification of research
Based on the:
– Goal: Basic & applied
– Specific objectives: Descriptive & explanatory,
– Approaches: Qualitative & quantitative
– Designs: experimental & non-experimental
– Type of data used: Primary & secondary
– Fields of study: natural science, social science,
educational, behavioral science, health science,
etc

5. Study design
Observational Experimental
Descriptive Analytic RCT CIT
Case study
Case series
Ecology
Cross-sectional
Trend study
Case-control
Cohort

6. Descriptive Studies
Descriptive Studies
• Use to describe occurrence of disease or health
related problems
– Prevalence of a disease, mortality, etc
– Rate of certain behaviour

7. Descriptive Studies
• Use to describe the distribution of disease by
– Time:
• When does the disease occur commonly or rarely,
or
• Is the frequency of the disease now different from
the corresponding frequency in the past?
– Person: Age, sex, race
– Place: Where are the rates of disease highest and
lowest

8. Person
Place
Time
Cases
0
5
10
15
20
25
1 2 3 4 5 6 7 8 9 10
Describe
Who? Where? When?

9. Descriptive studies
• Describing these factors does not link to any thing
• However they are:-
– Useful to identify unusual distributions or correlations
– can be used to generate interesting hypothesis
– Useful for health planning
• The most frequent designs strategy found in the
epidemiologic literature

10. Types of descriptive study
Types of descriptive study
1. Case report
2. Case series
3. Ecological descriptive studies
4. Trend studies
5. Cross-sectional surveys:

11.  It study the health profile of a single individual
(ex. detailed report of single patient) using
a careful and detailed report by one or more
clinicians.
 It is common form that is published in articles
 It is constructed using
 Simple history,
 Physical examination and
 Lab / radiologic investigation.
1. Case reports

12. Reports a series of cases of a specific condition, or a series of
treated cases.
 Individual case report can be expanded to a case series, which
describes characteristics of a number of patients (usually 5-12)
with a similar disease.
 Similar to case report, it is usually made on cases having new and/
or unusual disease (giving interest to clinicians)
 It is often used an early means of identifying the presence of
epidemic or to detect the emergence of new disease
2. Case series
2. Case series

13. Example: The first five AIDS cases in USA.
Five young, previously healthy homosexual men were
diagnosed as having Pneumocystis carinii pneumonia at
Los Angeles hospital during a six month period from 1980
to 1981.
This form of pneumonia had been seen almost exclusively
among older people whose immune systems were suppressed.
This unusual circumstance suggested that these individuals
were actually suffering with a previously unknown disease,
subsequently it was called AIDS.
13
Case reports

14. 3: Ecological descriptive study
3: Ecological descriptive study
The unit of observation is an
 aggregate (ex. family, clan or school) or
an ecological unit (a village, town or country)
 An ecologic study evaluates group characteristics in a
population.
The data used do not match any single individual to
both risk factor & outcome,

15. Ecological/Correlationals tudy
 Uses aggregated data from entire population to compare
disease frequencies.
 Can be done quickly and inexpensively, often using already
available data.
 The aggregate data could be
 Prevalence of a health event,
 Death rate,
 Incidence of a health related problem
 Cannot demonstrate a causal relationship but suggests areas for
further research,

16. Example
 Fluoride content of water and dental caries
– Proportion of people with dental caries in villages
Vs
– Fluoride content of water in villages

17. 4: Trend studies
Data may be collected at different points in time & changes in
the pattern are analyzed.
Though different study subjects are studied at each time, each
sample can represent the same type of population.
Trend studies often involve long period of data collection.
In most cases, the same researcher does not personally collect
the data,
Secondary analysis of data collected over time by several other
researcher/observers or routinely collected data can be used.

18. 5. Cross-sectional studies or surveys
 Called study of prevalence
 Survey is conducted in a population, to find prevalence of a
disease & exposure.
 Exposure and disease status are assessed
simultaneously among individuals at the same point in
time .
 Data are obtained only once.
 May be used first step in longitudinal or case control
studies.

19. Cont…
 Useful to assess presence of an association, (could be
analytic design).
 Measures of association is made using odds ratio.
 Since exposure & disease status is assessed at a single
point in time, temporal relationship b/t exposure & disease
can not be clearly determined.

20. Cross-sectional studies …
Ex: Surveys in which the distribution of a disease,
disability, & nutritional status is assessed.
-KAP survey: A common procedure used in FP, HIV/AIDS,
or on other services
-Health systems research to describe ‘prevalence’ by
certain characteristics – pattern of health service utilization
and compliance – or in opinion surveys.

21. Advantages Disadvantages
 Quick & cheep.
 Provide prevalence
information.
 Researcher has control over
 the selection of study units.
 the measurements used.
 Can study several factors &
outcomes at one time.
 Provides early clues for
hypothesis generation i.e.
First step for analytic study.
 Does not indicate temporal
relationship b/t exposure &
outcome hence cause &
effect associations
associations can’t
be established.
 Potential bias
bias in
 measuring
measuring exposure
 Sampling
Sampling bias
bias.
 Does not yield incidence
incidence
or true relative risk
relative risk.

22. Analytic epidemiological studies
Analytic epidemiological studies

23. Types of analytic epidemiological studies
Types of analytic epidemiological studies
There are two types
There are two types
1. Observational studies
1. Observational studies
a. Case-control study
a. Case-control study
b. Cohort study
b. Cohort study
2. Experimental / intervention studies
2. Experimental / intervention studies

24. Observational studies
Observational studies
exposure outcome cohort
effect-cause
case-control
cause-effect
cross-sectional
exposure outcome

25. a. Case control study
a. Case control study
 Relatively simple and economical
Relatively simple and economical
 Better for rare diseases
Better for rare diseases
 They include people with a disease of interest and a
They include people with a disease of interest and a
suitable control group (with out a disease)
suitable control group (with out a disease)
 The occurrence of possible cause is compared between
The occurrence of possible cause is compared between
cases and their controls.
cases and their controls.

26. Design of case control
Design of case control
Exposed
Non-exposed
Exposed
Non-exposed
Cases
(People with
disease)
Controls
(People without
disease)
Population
Time
Direction of inquiry

27. Disease
No disease
Exposure
?
?
Retrospective Nature
Case-Control Study
(Case)
(Control)

28. Measures of association
Measures of association
 The association of exposure to a disease is measured by
The association of exposure to a disease is measured by
calculating the
calculating the odds ratio
odds ratio.
.
 It is calculated by ratio of the odds (chance) of
It is calculated by ratio of the odds (chance) of
exposure among the cases to the odds of exposure
exposure among the cases to the odds of exposure
among controls
among controls
 Odds ratio is similar to risk ratio, particularly for rare
Odds ratio is similar to risk ratio, particularly for rare
disease
disease
OR =
OR =
Odds of exposure among cases
Odds of exposure among cases
Odds of exposure among controls
Odds of exposure among controls

29. Odds Ratio Calculation
Odds Ratio Calculation
Odds of Exposure in case =
Proportion of exposure among cases
Proportion of non-exposed among cases
Odds of Exposure in control =
Proportion of exposure among controls
Proportion of non-exposed among controls
Odds ratio =
Odds of exposure in Case
Odds of Exposure in Control

30. Example:
☻A case-control study conducted to learn the use of oral
contraceptives on the development of myocardial infarction
in an equal number of cases and control drawn from a two
thousand population in Butagira town.
☻The result showed that the prevalence of contraceptive
use was 69.3% and 32% among cases and control groups
respectively.
☻Define the association?

31. Oral Myocardial Infarction
Contraceptive Cases Controls OR
Yes 693 320 ?
No 307 680 Ref.
Total 1000 1000
% exposed 69.3% 32 %
Odds = a/c b/d
Odds ratio = a/c = ad
b/d cb

32. Oral Myocardial Infarction
Contraceptive Cases Controls OR
Yes 693 320 4.8
No 307 680 Ref.
Total 1000 1000
% exposed 69.3% 32 %
The odds of myocardial infarction is 4.8 times higher
among women using oral contraceptives when
compared to non-users.

33. b. Cohort study
b. Cohort study
 It is also called
It is also called follow up study
follow up study.
.
 The word cohort = a group of people who share a
The word cohort = a group of people who share a
common experience.
common experience.
A Birth cohort, a cohort of smokers, a cohort of
A Birth cohort, a cohort of smokers, a cohort of
2010 graduates, etc.
2010 graduates, etc.
 It measures the incidence of disease among exposed &
It measures the incidence of disease among exposed &
non-exposed population.
non-exposed population.

34. Cohort study
Cohort study
 It tries to look occurrence of a disease from an exposure
It tries to look occurrence of a disease from an exposure
(rare exposure).
(rare exposure).
 They include people
They include people exposed
exposed to a certain condition &
to a certain condition &
people
people not exposed
not exposed to such condition.
to such condition.
 The occurrence of a disease (
The occurrence of a disease (outcome
outcome) is compared
) is compared
between exposed and non-exposed people.
between exposed and non-exposed people.

35. Population
People
with
out a
disease
Exposed
Not -Exposed
Disease
Disease
No disease
No disease
Direction of inquiry
Time
Cohort study design
Cohort study design

36. Measures of association
Measures of association
 The association b/t exposure & a disease is measured
The association b/t exposure & a disease is measured
by calculating the
by calculating the Relative (risk) ratio
Relative (risk) ratio.
.
 It is calculated by ratio of the incidence of disease
It is calculated by ratio of the incidence of disease
among exposed to the incidence of the disease among
among exposed to the incidence of the disease among
non-exposed.
non-exposed.
 Association is measured directly.
Association is measured directly.
RR =
RR =
Incidence of disease among exposed
Incidence of disease among exposed
Incidence of disease among non-exposed
Incidence of disease among non-exposed

37. Advantages
Advantages
 Temporal relationship between exposure and
Temporal relationship between exposure and
outcome well ascertained.
outcome well ascertained.
 It assesses more the effects of rare exposure.
It assesses more the effects of rare exposure.
 Occupational settings
Occupational settings
 It allows to measure multiple effects of a single
It allows to measure multiple effects of a single
exposure.
exposure.

38. Limitations
Limitations
 Time consuming and expensive.
Time consuming and expensive.
 Bias associated with loss to follow up.
Bias associated with loss to follow up.

39. 2. Experimental /
2. Experimental /
intervention studies
intervention studies

40. Experimental study
Experimental study
 It involves manipulating a variable in one or more
It involves manipulating a variable in one or more
groups of people
groups of people.
.
 Intervention or manipulation of the population by
Intervention or manipulation of the population by
the investigator exists.
the investigator exists.
e.g.
e.g. Elimination of a dietary factor to cause allergy.
Elimination of a dietary factor to cause allergy.
Testing a new treatment on a selected group of pts.
Testing a new treatment on a selected group of pts.
 The effect of intervention are measured by
The effect of intervention are measured by
comparing the outcome in the experiment group
comparing the outcome in the experiment group
with the control group.
with the control group.
 It is usually prospective (has temporal relationship)
It is usually prospective (has temporal relationship)

41. Measures of association
Measures of association
 The association b/t exposure & a disease is
The association b/t exposure & a disease is
measured by calculating the
measured by calculating the Relative (risk) ratio (as
Relative (risk) ratio (as
in cohort study)
in cohort study).
.
 Association is measured directly.
Association is measured directly.
RR =
RR =
Incidence of disease among exposed
Incidence of disease among exposed
Incidence of disease among non-exposed
Incidence of disease among non-exposed

42. Three types of Experimental study
Three types of Experimental study
1. Randomized controlled clinical trial
1. Randomized controlled clinical trial
2. Field trial
2. Field trial
3. Community trial
3. Community trial
Limitations
Limitations
 Time consuming and relatively very expensive
Time consuming and relatively very expensive
 Ethical issues are given high consideration.
Ethical issues are given high consideration.