In brief about case control and cohort study used commonly in epidemiology.

1. Case control study and Cohort
study
NIVETHA C

2. OBSERVATIONAL STUDIES
• DESCRIPTIVE STUDIES
• ANALYTICAL STUDIES
1. Ecological
2. Cross-Sectional
3. Case- Control
4. Cohort

3. CASE CONTROL STUDY
 Retrospective study
Features:
1. Both exposure and outcome have occurred before the start
of the study
2. The study proceeds backwards from effect to cause
3. It uses a control or comparison group to support or refute
an inference

4. Basic Steps
1. Selection of cases and controls
2. Matching
3. Measurement of exposure
4. Analysis and interpretation

5. Selection of cases and controls
• SELECTION OF CASE :
The prior definition of what constitutes a case is crucial to
the case control study . It involves 2 specifications
1. Diagnostic criteria
2. Eligibility criteria
• SOURCE OF CASES
Cases can be drawn from hospitals or general population.

6. • SELECTION OF CONTROLS:
THE CONTROLS MUST BE FREE FROM DISEASE UNDER STUDY
They must be similar to the cases as possible except for the
absence of the disease under study .
SOURCE OF CONTROLS:
Hospitals
Relatives
Neighborhood
General population
One control per case if many cases are available or as many as 2,3
, or even 4 if the study group is small

7. • MATCHING :
Defined as the process by which we select controls in such a way
that they are similar to cases with regard to certain pertinent selected
variable which are known to influence the outcome of disease and which
if not adequately for comparability could distort of confound the result .
• A confounding factor is defined one which is associated with both
associated disease and is distributed un equally in study and control
groups .

8. • Types
1. Group Matching: assigning cases to
subcategories based on their characteristics like agem
occupation, etc. and then establishing appropriate
controls.
2. Pair matching: It is finding a control for
particular case as closely resembling as possible except
for disease under study.

9. • MEASUREMNT OF EXPOSURE :
Information of exposure should be obtained precisely in same
manner in both the cases and controls.
It can be recorded by interview, by questionnaires or by
studying past records.

10. • ANALYSIS :
t is the final step to find out :
1. Exposure rate among cases and controls to a suspected
factor.
2. Estimation of diseases risk associated with exposure
EXPOSURE RATES
A case control study provides a direct estimation of the exposure to
a suspected factor ion the disease and non-disease groups

11. Exposure rates
Cases :
𝑎
𝑎+𝑐
Controls :
𝑏
𝑏+𝑐
The next step is to ascertain
weather there is a statistical
association between exposure
status and disease occurrence
which is resolved by
calculating p value.
Suspected or
risk factors
Cases
(disease
present)
Controls
(Disdease
Abest)
Present a b
Absent c d
a+c b+d

12. ESTIMATION OF RISK :
It is obtained by an index known a ‘ relative risk ‘ or odds ratio
• 𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑟𝑖𝑠𝑘 =
𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑎𝑚𝑜𝑛𝑔 𝑒𝑥𝑝𝑜𝑠𝑒𝑑
𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑎𝑚𝑜𝑛𝑔 𝑛𝑜𝑛 𝑒𝑥𝑝𝑜𝑠𝑒𝑑
• Odds ratio is the measure of the strength of association
between the risk factor and outcome .it based on 3
assumptions :
1. disease being investigated must be relatively rare .
2. case must be representative of those with the disease
3. controls mist be representative of those withput the
disease.
odds ratio = ad / bc.

13. ADVANTAGES
• Relatively easy to carry out
• Rapid and inexpensive compared to
cohort study
• Requires comparatively few
subjects
• No risk to the subject
• Allows the study of several different
etiological factors like effect of
smoking, physical activity, diet.
DISADVANTAGES
• Problems of bias
• Selection of an appropriate control
group may be difficult
• Incidence cannot be measured, only
relative risk can be estimated
• Not suited for evaluation of therapy
or prophylaxis of disease

14. COHORT STUDY:
• Definition: it is a type of study usually undertaken to obtain
additional evidence to refute or support the existence of an
association between suspected cause and disease.
• Characteristics
1. Cohorts are identified prior the appearance of the disease
under investigation .
2. The study groups are observed over a period of time to
determine the frequency of the disease among them .
3. The study proceeds forward from cost to effect

15. • COHORT SELECTION :
Cohort is a group of peple who share a common characteristic or
experience within a definite time period.
• For selecting a cohort the following facts should be considered
1. Cohorts must be free form the disease under study.
2. Both the study and control cohorts should be equally
susceptible to the disease under study.
3. Both the cohort must be comparable in respect to all
possible variable which may Influence the frequency of
the disease.

16. Cohort Disease
Present
Disease
Absent
Total
Exposed to etiological agent a b a+b
Not exposed to etiological agent c d c+d
Framework of cohort study

17. TYPES OF COHORT STUDY
COHORT
STUDIES
PROSPECTIVE RETROSPECTIVE
COMBINATION
OF BOTH.

18. STEPS OF COHORT STUDY :
• Selection of study subjects
• Obtaining data on exposure
• Selection of comparison groups
• Follow up
• Analysis

19. • SELECTION OF STUDY SUBJECT :
Subjects of a cohort study are usually assembled from general
population or special groups .
• OBTAINING DATA ON DATA EXPOSURE :
Information about exposure is obtained directly from any one or
all of the following sources :
Cohort members , review of records , medical examinations
or special tests and environmental surveys

20. • SELECTION OF COMPARISON GROUPS :
There are many ways of assembling a group .
1. Internal comparison : when a single cohort enters the
study, on the basis of information obtained, be classified
into several comparison groups according to the degrees of
levels of exposure to risk before the development of disease in
question.
2.External comparison : when the degree of comparison is
not available , it is necessary to put up an external , to
evaluate the experience of the exposed group .
3.Comparison with general population rates .

21. 4. FOLLOW UP COMPRISES OF:
1.Period medical examination of each member if the cohort
2.Reviewing physician and hospital records
3.Routine surveillance of death records
4.Mailed questionnaires , telephone calls and periodic home
visits.
• Limitation :
A certain percentage of losses to follow up are inevitable due to
death , change of residence , migration , or withdrawal of
occupation .
These losses may bias the result .

22. • ANALYSIS:
- Incidence rates
- Estimation of risks

23. • INCIDENCE RATES:
Among cases:
𝑎
𝑎+𝑏
Among controls :
𝑐
𝑐+𝑑
• ESTIMATION OF RISK
-Relative risk
-Attributable risk
-Absolute risk

24. • Relative Risk:
It is the direct measure of strength of the association
between suspected cause and effect.
• Relative risk:
𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑜𝑓 𝑑𝑖𝑠𝑒𝑎𝑠𝑒 𝑎𝑚𝑜𝑛𝑔 𝑒𝑥𝑝𝑜𝑠𝑒𝑑
𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑜𝑓 𝑑𝑖𝑠𝑒𝑎𝑠𝑒 𝑎𝑚𝑜𝑛𝑔 𝑛𝑜𝑛−𝑒𝑥𝑝𝑜𝑠𝑒𝑑
Interpretation: I exp= I nonexp = no correlation
I exp> I nonexp = Positive association
I exp < I nonexp = negative correlation

25. • Attributable Risk: It is the difference in incidence rates of disease
between an exposed group and non exposed group
• =
𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑜𝑓 𝑑𝑖𝑠𝑒𝑎𝑠𝑒 𝑟𝑎𝑡𝑒 𝑎𝑚𝑜𝑛𝑔 𝑒𝑥𝑝𝑜𝑠𝑒𝑑+𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑜𝑓 𝑑𝑖𝑠𝑒𝑎𝑠𝑒 𝑟𝑎𝑡𝑒 𝑎𝑚𝑜𝑛𝑔 𝑛𝑜𝑛 𝑒𝑥𝑝𝑜𝑠𝑒𝑑
𝑡𝑜𝑡𝑎𝑙 𝑠𝑡𝑢𝑑𝑦 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛
x100
• It signifies potential for prevention.

26. • Absolute risk:
Risk of developing the disease, irrespective of exposure
to risk factor and is expressed in percentage.
• =
𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑎𝑚𝑜𝑛𝑔 𝑒𝑥𝑝𝑜𝑠𝑒𝑑+𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑐𝑒 𝑎𝑚𝑜𝑛𝑔 𝑛𝑜𝑛 𝑒𝑥𝑝𝑜𝑠𝑒𝑑
total study population
x100

27. ADVANTAGES
• Incidence can be calculated
• Several possible outcomes
related to exposure can be
studied simulateously
• Provide a direct estimation of
relative risk
• Dose-response ratios can also
be calculated
• DISADVANTAGES
• Involve a large number of
people
• General unsuitable for
investigating uncommon
disease or diseases with low
incidence in the population
• Long time to complete study
• Loss of substantial proportion
of cohort due to migration,
loss of interest.
• Expensive

28. Thank you