1. By Dr Aijaz Ahmed Sohag
Prep by: Abdul Wasay Baloch
Amna Inayat Medical College
Case Control vs Cohort Control

2. Case Control Cohort Control
 Proceeds from effect to
cause
 Starts with the disease
 To know suspected cause
occurs more frquently
having disease than those
without disease
 Suitable for study of Rare
disease
 Relative inexpensive
 Proceeds from cause to
effect
 Starts with exposure
 To know whether disease
occur more frequently in
those exposed to risk
factors(a+b) than non
exposed (c+d)
 Suitable for exposure of
Rare response
 Comparatively expensive
Case Control Cohort Control

3.  Generally yields only
estimate risks (odds ratio)
 Time of study relatively
short
 Population size needed
relatively small
 Potential bias larger
(assessment of exposure)
 Generally yeilds relative
risk, attributable Risk,
besides incidence rate
 Relatively large
 Comparatively mimic less
(assessment of outcomes)
 E.g.
 Smoking and lung cancer.
Fermingham heart study, Oral
contraceptive and health
Case Control Cohort Control

4.  RR estimate may increase or decrease as a result of bias
a) Bias due to confounding (may be reduced by Matching)
b) Memory or Recall bias
c) Selection bias
d) Berkesonian bias or Joseph bias
e) Interviewer bias (may be reduced by double blinding)
 Examples
 Adenocarcinoma of Vagina
 Oral contraceptive and thromboemolic disease
 Thalidomide tragedy
 Ethical problems minimal
 Incidence can not be measured, and can only estimate the
Relative Risk

5. Incidence Rate
Cigrette smoking Developed Lung
cancer
Did Not Develop
Lung cancer
Total
Yes 70 (a) 6930 (b) 7000 (a+b)
No 3 (C ) 2997(D) 3000 (c+d)

6.  Incidence Rate among Smokers a/(a+b) = 70/7000*1000
= 10 per 1000
 Among Non- smokers = c/(c+d)= 3/3000*1000
= 1 per 1000
 Estimation of Risk:
 Relative risk = incidence of dis among exposed/incidence of
dis among non expose= RR = 10/1=10
 Attributable risk= incidence of dis among exposed – incidence
of disease among non exposed/incidence rate among exposed
=10-1/10 *100 = 90%

7. RR AR
 To study etiology (cause) it has an edge
over AR
 It measure Strength of association
between suspected cause and effect
 Larger the RR, stronger the association
between Cause and Effect. RR 1
indicates no association
 RR has less public health importance
as does RR
 E.g. if RR is 10, it means smokers are
10 times at greater risk of developing
lung cancer than non smokers.
 Comparatively AR has less
importance studying antilogy of
disease
 It measures To What extent disease
under study Attributed to exposure
e.g. 90 % lung cancer in smokers
was due to smoking
 AR gives better idea than does RR
of the impact of preventive/public
health program in reducing problem