2. Objectives
Understand the concept of epidemiological
studies.
Understand the concepts of exposure,
outcome and risk.
Identify the association between exposure
and outcome.
Quantify the magnitude of association by
calculating and interpreting the measures
of association:
Relative risk or risk ratio
Odds ratio
2
3. Important study questions
1. What? 1. Event Surveillance data
sources
2. When? 2. Time
Surveillance,
3. Where 3. Place
? Descriptive studies
4. Person
4. Who?
5. Reason
Why? Analytical studies
5. 6. Way
6. How? 3
4. Concepts
Variable
Outcome or Effect variable
Measures of disease
Risk Factor or Exposure
variable
Measures of risk
Association of risk and
disease
Measures of association
4
5. Types of Study designs
Non Intervention Studies
Descriptive studies
Comparative (analytical) studies
Exploratory studies
Intervention Studies
The two categories of intervention studies are:
Experimental studies
Quasi-experimental studies.
5
6. Current
Classification
E P ID E M IO L O G IC A L S T U D Y D E S IG N S
D E S C R IP T IV E A N A L Y T IC A L
1 . C a s e R e p o rt O B S E R V A T IO N A L E X P E R IM E N T A L
2 . C a s e S e rie s
CASE CONTROL 1 . S in g le B lin d
3 . C o rre la tio n / E c o lo g ic a l
COHORT 2 . D o u b le B lin d
4 . C ro s s S e c tio n a l / P re v a le n c e
3 . T rip le B lin d
6
7. E p id e m io lo g ic a l S t u d ie s
I n te r v e n tio n a l s tu d ie s N o n -in te r v e n tio n a l S tu d ie s
o r O b s e r v a tio n a l
C lin ic a l F ie ld C o m m u n ity E x p lo r a to r y D e s c r ip tiv e
T r ia ls I n v e s tig a tio n I n te r v e n tio n
C a s e s e r ie s C r o s s -s e c tio n a l s u r v e y s
A n a ly tic a l
A n a ly tic a l S tu d ie s
C r o s s -s e c tio n a l C a s e -C o n tr o l C ohort
c o m p a ra tive
9. Descriptive Studies
Descriptive studies involve the
systematic collection and
presentation of data to give a
clear picture of a particular
situation and can be carried out
on a small or large scale.
Case studies
Case series
9
10. Comparative or Analytical
Studies
An ANALYTICAL STUDY attempts
to establish causes or risk factors
for certain problems. This is done
by comparing two or more groups,
some of which have or develop
the problem and some of which
have not.
10
11. Cross-sectional
Comparative studies
Many cross-sectional surveys focus on
comparing as well as describing
groups.
For example, a survey on malnutrition
may wish to establish:
The percentage of malnourished
children in a certain population
Socio-economic, physical, political
variables that influence the availability
of food 11
12. Cross Sectional
Surveys
Quantify the distribution of
certain variables in a study
population at a point of time.
They may cover, for example:
Physical characteristics of people,
materials, or the environment,etc
12
13. Cross Sectional
Surveys
The behavior of people and the
knowledge, attitudes, beliefs, and
opinions that may help to explain that
behaviour (KAP studies), or events that
occurred in the population.
Cross-sectional surveys cover a sample
of the population. If a cross-sectional
study covers the total population it is
called a census.
13
14. Case Control
Studies
In a CASE-CONTROL STUDY, the
investigator compares one group
among whom a problem is (e.g.,
malnutrition) with another group,
called a control or comparison
group, where the problem is
absent to find out what factors
have contributed to the problem.
14
16. Odds Ratio (OR)
Measure of the strength of the
association between risk factor and
outcome.
The derivation of the Odds Ratio is
based on three assumptions:
- The disease being investigated must be
relatively rare.
- The cases must be representative of
those with the disease
- The controls must be representative of
those without disease
16
17. A study was conducted to find out
the association of smoking to
lung cancer. 100 cases of lung
cancer were interviewed about
their smoking status and 60 of
them were smokers. 200 Normal
people were also interviewed and
40 of them were smokers. Find
the odd ratio in the given
scenario and interpret your result
17
as well.
18. Biopsy Results
Smoking CA Lung CA Lung
Total
status Positive Negative
a+b
Yes a b
c +d
No c d
a +b +c + d
Total a+c b+d
18
19. Biopsy Results
Smoking CA Lung CA Lung
Total
status Positive Negative
a+b
Yes 60 40
c +d
No 40 160
a +b +c + d
Total 100 200
19
20. Odd ratio = a/c ÷ b/d
= a/c x d/b
= 60 x 160
40 x 40
= 6
Interpretation:
Lung cancer patients are six times
more likely to be smokers than normal
persons
20
21. Cohort Studies
In a COHORT STUDY, a group of individuals
that is exposed to a risk factor (study group)
is compared with a group of individuals not
exposed to the risk factor (control group).
The researcher follows both groups over time
and compares the occurrence of the problem
that he or she expects to be related to the
risk factor in the two groups to determine
whether a greater proportion of those with
the risk factor are indeed affected.
21
22. What is a cohort?
Cohort - Latin word for one of the 10
divisions of a Roman legion
A group of individuals
sharing same experience
followed up for a specified period of time
Examples
birth cohort
occupational cohort chemical plant workers
22
24. Cohort Study
Disease
Exposure Study starts occurrence
time
24
25. Cohort Study
Disease
Study starts Exposure occurrence
time
25
26. Cohort Study
Not
exposed Incidence among
Non exposed
exposed
Incidence among
exposed
26
27. Steps in designing Cohort
Studies
Identify group of exposed subjects
Identify group of non-exposed subjects
Follow-up both groups for disease
Measure frequency of disease
occurrence in both groups
Compare risks between exposed and
non-exposed group
27
28. Relative Ratio/Risk (RR)
Ratio of incidence of the disease
(or death)among exposed and the
incidence among non-exposed.
It is a direct measure (or index) of
the “strength” of the association
between suspected cause and
effect
28
29. A scientist wanted to study the effect of
smoking on lung cancer. He enrolled 500
people who were smoking and the 1000
individuals who were not smokers. After
15 years of follow up he found that
among the smokers 50 developed lung
cancer while among the non smokers 25
developed lung cancer. Find the relative
risk of lung cancer among smokers and
interpret the result as well
29
30. Biopsy Results
CA Lung CA Lung
Smoking status Total
Positive Negative
a+b
Yes a b
c +d
No c d
a +b +c + d
Total a+c b+d
30
31. Biopsy Results
CA Lung CA Lung
Smoking status Total
Positive Negative
500
Yes 50 b
1000
No 25 d
a +b +c + d
Total a+c b+d
31
32. Relative risk = Incidence of disease Among
Exposed Incidence of disease among
non exp
RR = a/a+b ÷ c/c+d
= 50/500 ÷ 25/1000
= 50/500 x 1000/25
= 4
Interpretation:
Smokers are 4 times more likely to
develop lung cancer than non smokers
32
33. Exploratory Studies
An Exploratory study is a small-
scale study of relatively short
duration, which is carried out
when little else is known about a
situation or a problem.
33
34. Intervention Studies
In intervention studies, the
researcher manipulates a
situation and measures the
effects of the manipulation.
Usually (but not always) two
groups are compared, one in
which the intervention takes
place (e.g.. treatment with a
certain drug) and another group
that remains "untouched" (e.g.,
treatment with a placebo) .
34
35. Experimental Studies
Experimental design is the only type of
study design that can actually prove
causation.
In an EXPERIMENTAL STUDY, individuals
are randomly allocated to at least two groups.
One group is subjected to an intervention or
experiment, while the other group(s) is not.
The outcome of the intervention (effect of the
intervention on the dependent
variable/problem) is obtained by comparing
the two groups.
35
36. Experimental studies
Exposed
Exposure Disease
assigned occurrence
Not exposed
Unethical to perform experiments on people if
exposure is harmful
36
37. Past Present Future
Case-control study
Exposure Outcome
Prospective Cohort study
Exposure Outcome
Retrospective Cohort study
Exposure Outcome
Cross-sectional study
Exposure
Outcome
38. Classical Experimental
Study Design
The classical experimental study
design has three characteristics:
Manipulation
Control
Randomization
38
39. Quasi – Experimental
Studies
In a QUASI-EXPERIMENTAL STUDY,
at least one characteristic of a true
experiment is missing.
One of the most common quasi-experimental
designs uses two (or more) groups, one of,
which serves 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.
39
40. Tutorial
An epidemiologist wants to find out the
association of bottle feeding with the
incidence of diarrhoea.
He enrolled 100 children fed through
bottle and 80 experienced at least one
episode of diarrhoea during the follow up
period. Out of 150 on breast feeding 60
had experienced diarrhoe during the
same period.
Calculate the relevant measure of risk
Interpret your result
40