1. Study Designs
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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
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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
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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.
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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
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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

8. Association between exposure
and outcome
Exposure Outcome
Cause Effect
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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
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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.
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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
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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.
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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.
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15. 15

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
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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
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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
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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
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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
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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.
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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
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23. 23

24. Cohort Study
Disease
Exposure Study starts occurrence
time
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25. Cohort Study
Disease
Study starts Exposure occurrence
time
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26. Cohort Study
Not
exposed Incidence among
Non exposed
exposed
Incidence among
exposed
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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
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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
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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
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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
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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
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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
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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.
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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) .
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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.
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36. Experimental studies
Exposed
Exposure Disease
assigned occurrence
Not exposed
Unethical to perform experiments on people if
exposure is harmful
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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
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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.
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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
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41. Thank you
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