Basic Civil Engineering first year Notes- Chapter 4 Building.pptx
Cohort and case con revised
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