1. CASE CONTROL
STUDIES
DR MUHAMMAD TAUSEEF JAVED
MBBS.DPH.FCPS(CM)M.Phil(CM).Dip-Card
2. CASE CONTROL STUDIES
Finding Risk Factor/ Cause of Disease
Cases Control
Comparison of Diseases verses non-disease
3. Case Control Study design
Exposure Present
Diseased
Exposure absent
Compare Trace back Select
Exposure Present
Non-diseased
Exposure absent
Case control study proceed from effect to the cause
4. Case Control Study Steps
Definition of cases
Selection of cases
Selection of Controls
Ascertainment of exposure status
Analysis
Conclusion and interpretation
5. Defining the cases
How will you define poliomyelitis?
What is acute flaccid paralysis (AFP)
Is AFP a case of Poliomyelitis
What is WHO Criteria of poliomyelitis?
Case definition of Tuberculosis
Cases should be representative of all
diseased
6. Selection of Cases
Preferably new cases or incident cases
Prevalent cases (cases already present)
Random selection
Convenient selection
7. Control
Control must be Ideally matching
with the cases by age, sex and
other characteristics except the
control must not be suffering from
diseases
8. Criteria for control
Matching by all respect with cases
except not having disease
Hospital controls
Neighborhoods controls
Friends and relatives
Controls from general population
9. Methods of selection of control
Random Selection
Convenient Selection
What should be the ratio between
cases and control
10. Ascertainment of Exposure
1. Setting the criteria for exposure
2. Methods of exposures
Observation
Interviews
Questionnaire
Examination of records
11. Analysis
1. Exposure Rates among the
disease (Cases)
2. Exposure Rates among the non-
disease (Control)
3. Odd’s Ratio
12. 2 x 2 Table for Case Control Study
Design
Disease Status
Cases Control Total
Exposur
e Status Exp. a b a+b
Yes Total Exp
Exp. No c d c+d
Total non-
Esp
Total a+c b+d a + b+ c+
Total Dis. Total non- d
Dis. Grand total
13. Exposure Rates
a
Exposure Rates
among cases
/disease =
a+c
b
Exposure Rate
among the control
/non-disease =
b+d
14. Risk Ratio
a/b
Odds Ratio =
c/d
ad
Odds Ratio =
bc
15. Probability that an event will happen
Odds=
Probability that the even will not happen
Probability that an event will
happen
Odds=
1 - (Probability that the event
will happen)
16. Case control study
Odds of exposure among cases =
Cases Controls Probability to be exposed among cases
Probability to be unexposed among cases
Exposed a b a / (a+c)
Not exposed c d
Odds Ecases = ------------ = a / c
c / (a+c)
Total a+c b+d Odds of exposure among controls =
Probability to be exposed among controls
Probability to be unexposed among controls
b/ (b+d)
Odds Econtrols = ------------ = b / d
d/ (b+d)
a/c
OR = ---- = ad / bc
b/d
17. Case control study design
Cases Controls
Odds ratio
E a b
a b axd
---- --- = --- ----
c d bxc
E c d
18. Interpretation of Odds Ratio
What do you understand by OR= 1
?
What do you understand by OR= 3
?
What is your opinion if OR is less
than 1
19. Brain Storming
Can we estimate Incidence of disease
in case control study design?
Can we estimate time relationship of
cause and disease/effect?
What is the time relationship of cause
and effect?
20. Limitations
Is inefficient design for evaluation of
rare exposure
Incidence rate cannot be computed
Time relationship cannot be estimated
This study design is more prone to bias
21. Scope of case control study
design
It is quick and inexpensive as
compared to other analytic study
designs
It is particularly suitable for evaluation
of diseases with long latent periods
It is optimum design for rare diseases