This document discusses and compares case-control and cohort studies in epidemiology. It defines epidemiology as the study of health-related states in populations and applying this to control health problems. Analytical epidemiology focuses on testing hypotheses about individuals within populations. Both case-control and cohort studies are described as types of analytical epidemiology. Case-control studies are retrospective while cohort studies are prospective. The key differences and advantages/disadvantages of each study type are outlined.

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Presented by:
reMAN dhaKAL
CODSH-NMC
FIRST BATCH

2.  Introduction
 Types of epidemiology
 Types of analytical epidemiology
 Case control study
 Cohort study
 Comparison between case control and cohort
study
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3.  John M. last: "the study of the distribution
and determinants of health-related states in
specified populations, and the application of
this study to control health problems.
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EPIDEMIOLOGY

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Analytical epidemiology
 Second major type of epidemiology.
 Focus on individual within population unlike descriptive
epidemiology..
 Objective not to formulate hypothesis but to test
hypothesis.
TYPES
A. CASE CONTROL STUDY
B. COHORT STUDY

6.  Retrospective or trohoc study
 Distinct features:
1. Both exposure and outcome have occurred
before the start of disease
2. Study proceed backward from effect to
cause
3. Uses a control or comparison group to
support or refute an inference.
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The basic study design
Cases
(those with condition)
eg: cases with oral cancer
characteristic or risk factor)
Control
Unexposed (without
Eg. Non chewers
Exposed (with characteristic
(those without condition)
eg: those free of oral cancer
or risk factor)
Eg. tobacoo chewers

8. 1. Selection of cases and controls
2. Matching
3. Measurement of exposure , and
4. Analysis and interpretation.
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A. Selection of case
Definition of a case:
I). diagnostic criteria:
ii). Eligibility criteria
Sources of cases:
i). Hospital
ii). General population
B. Selection of control
Sources of controls:
i). Hospital controls(common
source of selection bias)
ii). Relatives
iii). General population
Number of
controls/control
groups

10.  Define as:”process by which we select controls in such a way
that they are similar to cases with regard to certain pertinent
selected variables(eg. Age) which are known to influence the
outcome of disease and which, if not adequately matched for
comparability, could distort or confounded the result”.
 CONFOUNDING FACTOR
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EXPOSURE
(eg. Consumption
of alcohol)
DISEASE
(eg. Oesophageal
cancer)
CONFOUNDING FACTOR
(eg. smoking, age)

11.  Definition and criteria about exposure are just as
important as those used to define cases and controls.
This may be obtained by :
 Interviews
 Questionnaries
 Studying past record of cases such as hospital
records, employment records etc.
 Clinical or laboratory examination.
Investigator should not know whether a subject is in
case or control group.
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12. The final step is analysis, to find out:
a) Exposure rates among cases and controls to suspected
factors
b) Estimation of disease risk associated with exposure
(ODD RATIO)
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13. Example of case control study of tobacco chewing and oral cancer
Tobacco
chewers
Non-chewers
Cases
(with oral cancer)
33 (a)
2 (c)
Controls
(without oral cancer)
55(b)
27(d)
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Total 35 (a +c) 82 (b +d)
EXPOSURE RATES
a. Cases =a/(a+c) = 33/35 = 94.2%
b. Controls =b/(b+d) = 55/82 = 67.0%

14. 1. Relative risk = Incidence among exposed
incidence among non exposed
= a c
(a+b) (c+d)
2. Odds ratio (OR) = (a/b)
(c/d)
 It measure strength of the association between
risk factor and outcome. ,

15. 1. Selection bias :
special types:
a) Prevalence incidence (selective survival)
b) Admission rate ( Berkson/Berkesonian)
2. Information bias:
a) Memory or recall bias
b) Telescopic bias
c) Interviewer’s bias
3. Bias due to confounding
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ADVANTAGES:
CASE CONTROL STUDY
1. Relatively easy to carry out.
2. Rapid and inexpensive
3. Require fewer subjects.
4. Suitable for investigation of
rare diseases.
5. No risk of subject.
6. Allows the study of several
different etiological factors.
7. Risk factor can be identify
8. No attrition problem
because do not require
follow up.
9. Minimal ethical problem.
DISADVANTAGES:
1. Problem of bias since it
relies on past memory or
past records.
2.Difficulty in selection of
appropriate control
group.
3. Can not measure
incidence only RR.
4. Doesn’t distinguish
between cause and
associated factors.
5.Not suited for the
evaluation of therapy or
prophylaxis of disease.

17.  Prospective ,longitudinal, incidence and
forward-looking study
 Distinguishing features:
a) The cohorts are identified prior to the
appearance of the disease
b) The study groups, so defined, are observed
over a period of time to determine the
frequency of disease among them
c) Study proceeds from cause to effect.
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18. time
Study begins here
Study
population
free of
disease
Factor
present
Factor
absent
disease
no disease
disease
no disease
present
future

19. When there is good association between
exposure and disease.
 When exposure is rare, but the incidence of
disease is high among exposed.
When attrition of study population can be
minimized.
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1. PROSPECTIVE COHORT STUDY
1. RETROSPECTIVE COHORT STUDY
2. A COMBINATION OF
PROSPECTIVE AND
RETROSPECTIVE COHORT STUDY.

21.  - Outcome has not yet occurred the time
of investigation begins.
Measure exposure
and confounder
variables
Exposed
Non-exposed
Outcome
Outcome
Baseline
time
Study begins here

22.  Outcomes have all occurred before the start
of the investigation.
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Measure exposure
and confounder
variables
Exposed
Non-exposed
Outcome
Outcome
Baseline
time
Study begins here

23. 1. Selecting of study subject
2. Obtaining data on exposure
3. Selection of comparison group
4. Follow up
5. analysis
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1. Selecting of study
subject
When exposure or cause of
death is fairly frequent in
the population
i. Select group –
Professional
group ( doctors,nurses )
ii. Exposure group-
High risk situation
(eg.radiologist exposed
to x-ray)
Obtaining data on
exposure
Information about exposure
may be obtained directly
from:-

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Selection of comparison
group
a. Internal comparisons:-
 Comparison groups are in built
(eg. Smoking, bp etc.) within
same cohort group.
b. External comparisons:-
 Eg. Smoker and non smoker,
radiologists with
opthalmologists.
c. With General population:-
 If none is available, mortality of
exposed group with general
population
Follow up
 Main problem
 Procedures to obtain data
for assessing the outcome
are:
a. Periodic medical checkup
b. Reviewing hospital records
c. Routine surveillance of
death records
d. Mailed questionnaries,
telephone calls, periodic
home visits.

26. Data are analysed in term of:
a. Incidence rates of outcome among exposed
and non-exposed:
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Data are analysed in term of:
a. Incidence rates of outcome among exposed
and non-exposed
RISK FACTOR
(TOBACCO)
5. ANALYSIS
DEVELOPED
ORAL CANCER
DID NOT
DEVELOP
TOTAL
PRESENT
(CHEWERS)
45 (a) 9955 (c) 10000 (a + c)
ABSENT
(NON CHEWERS)
5 (b) 9995 (d) 10000 (b + d)
Incidence rates:
1. Among tobacco chewers: = 45/10000 =4.5 per 1000
2.Among non chewers = 5/10000 = 0.5 per 1000

28. b. Estimation of risk
A. Relative risk (RR):
= incidence of disease among exposed
incidence of disease among non-exposed
= 4.5/0.5
= 9
 It implies 9 times higher risk of development
of oral carcinoma in tobacco chewers
compared to non chewers.
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29. B. Attributable risk (AR) Or “risk difference”:
Incidence of disease rate among
exposed- incidence among non exposed
Incidence rate among exposed
= 4.5 – 0.5
4.5
= 88.9%
 Indicates to what extent the disease under
study can be attributed to the exposure.
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30. 1. Selection bias:
 Non consent bias
 Missing data bias
2. information bias:
 Error in classification of individual
 Diagnostic bias
3. Confounding bias :
 Due to confounding factors
4. Post hoc bias:
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31. 1. Incidence can be calculated
2. Several possible outcomes related to
exposure can be studied simultaneously.
3. Provide a direct estimate of RR.
4. Dose response ratios can be calculated.
5. Since comparison groups are formed before
disease develops, certain forms of bias can
be minimized like misclassification of
individual.
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32. 1. Unsuitable for investigating uncommon disease.
2. Long time to complete study and obtain results.
3. Administrative problem –
 Extensive record keeping
4.Expensive
5. Alter people behavior
 Stop or decrease smoking
 Loss of interest
 migration
5. Ethical problem of varying important
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33.  Start disease :effect
cause
 First approach to test
hypothesis.
 Involve fewer subject.
 Yield result quickly.
 Suitable for studying rare
disease.
 Gives RR only.
 Start people: cause
effect.
 Reserved for testing precisely
formulated hypothesis.
 Involve larger number of
subjects.
 Results are delayed due to
long follow up.
 Unsuitable for studying for
rare diseases.
 Yield RR and AR.
 Relatively inexpensive.
 Expensive
 Does not give information
 Can give information more
about diseases other than that
than one disease.
selected for the disease. 33
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