Case control studies compare exposures in individuals with a disease (cases) to individuals without the disease (controls) to identify potential risk factors for the disease. The study involves selecting cases and controls, ascertaining their exposure status, analyzing differences in exposure rates using odds ratios, and interpreting results to determine if an exposure is associated with the disease. Key limitations are that it cannot estimate incidence rates or time relationships between exposure and disease.

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