3. Measuring of Risk
Introduction:
• Is there an association between Exposure and
Disease …?
• Do persons with exposure have higher level of
disease than persons without exposure…?
• Is the association real (causal)
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8. Measuring of Risk
Measures of association:
• The key to epidemiological analysis is
comparison.
• A measure of association quantifies the
relation ship between the exposure and disease
among the comparison groups.
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9. Measuring of Risk
Measures of association:
Examples of Measure of Association:
Risk Ratio (Relative Risk).
Odds Ratio.
Attributable proportion.
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10. Measuring of Risk
Measures of association: Risk Ratio
• Called also relative risk.
• Compares the risk of a health event among
one group with the risk among another group.
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11. Measuring of Risk
Measures of association: Risk Ratio(RR)
Risk Ratio (RR)=
Risk of disease in group of primary interest
Risk of disease in comparison group
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12. Measuring of Risk
Measures of association: Risk Ratio(RR)
RR less than 1.0
Indicates a decrease
risk for the exposed
group.
Indicating that
perhaps exposure
actually protects
against disease
occurrence.
RR of 1.0
Indicates identical
risk among the
two groups.
RR of more than
1.0
Indicates an
increase risk for
the group in the
numerator
usually the
exposed group.
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13. Measuring of Risk
Measures of association: Risk Ratio(RR)
Ill
Well
Total
Exposed
a
b
a+b
Unexposed
c
d
c+d
Total
a+c
b+d
a+ b+ c+ d
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14. Measuring of Risk
Measures of association: Risk Ratio(RR)
Example
In outbreak of T.B among prison inmates in South
Carolina in 1999, 28 of 157 inmates residing on the
east wing of the dormitory developed T.B, compared
with 4 of 137 inmates residing on the west wing.
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15. Measuring of Risk
Measures of association: Risk Ratio(RR)
T.B +
T.B -
Total
East Wing
a=28
b=129
a + b=157
West Wing
c= 4
d=133
c + d=137
Total
a+c=32
b+d= 262
a+ b+ c+ d=294
Risk of T.B among East Wing residents= 28/157 X100= 17.8%
Risk of T.B among West Wing residents=4/137X100= 2.9%
RR= 17.8/2.9= 6.1
The inmates who resided in the East wing of the dormitory were 6.1 times as likely to
develop T.B as those who resided in the West wing.
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16. Measuring of Risk
Measures of association: Risk Ratio(RR)
Example
In an outbreak of varicella in Oregon in 2002,
varicella was diagnosed in 18 of 152 vaccinated
children compared with 3 of 7 unvaccinated
children. Calculate the risk ratio..?
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17. Measuring of Risk
Measures of association: Risk Ratio(RR)
Varicella +
Varicella -
Total
Vaccinated
18
134
152
Unvaccinated
3
4
7
Total
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138
159
Risk of varicella among vaccinated children= 18/152 X100=11.8%
Risk of varicella among unvaccinated children= 3/7X100= 42.9%
RR=11.8/42.9=0.28
The vaccinated children were only approximately as one fourth to develop varicella
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as were unvaccinated children.
18. Measuring of Risk
Measures of association: Risk Ratio(RR)
RR is mostly used in Cohort Studies
Can be Used also in Cross sectional as
well as Clinical trails.
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21. Measuring of Risk
Measures of association: Odds Ratio(OD)
Is another measure of association that quantifies
the relation ship between an exposure with two
categories and health problem.
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22. Measuring of Risk
Measures of association: Odds Ratio(OD)
In statistics, an odds of an event is the ratio of:
the probability that the event WILL occur to
the probability that the event will NOT occur
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23. Measuring of Risk
Measures of association: Odds Ratio(OD)
In simpler term, an odds of an event can be
calculated as:
Number of events divided by number of non-events
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28. Measuring of Risk
Measures of association: Odds Ratio(OD)
Example
A case control study was conducted in Hamad
Hospital to compare 35 lung cancer cases to controls
to determine the associated factors related to lung
cancer. Out of 117 subjects in the sample, 88 were
daily smokers. Only two cases were found to be
nonsmokers.
Calculate the Odds Ratio and interpret it. ?
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29. Cases
Lung Cancer
Controls
Without Lung Cancer
Total
Daily Smokers
Nonsmokers
33
55
88
2
27
29
Total
35
82
117
Odds Ratio = 33X 27 / 2X55 = 8.1
Daily Smokers showed risk of having lung cancer 8.1 times that of
nonsmokers.
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33. Any relation between RR and OR ….?
Relative Risk
Odds Ratio
• Needs incidence of the disease. • Incidence not for its calculation.
• Usually in Prospective, cross
• Retrospective studies and in
sectional and clinical trail
cross sectional.
Studies.
• Good estimate for RR in case of
• demonstrates temporality.
low frequency disease.
• OR can overestimate risk, in
rare disease.
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34. Measuring of Risk
Measures of association: Attributable proportion
• Called also attributable risk percent.
• Is a measure of the public health impact of a
causative factor.
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35. Measuring of Risk
Measures of association: Attributable proportion
The calculation of this measure assumes that
occurrence of disease in the unexposed group
represent the baseline or expected risk for that
disease.
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36. Measuring of Risk
Measures of association: Attributable proportion
It assumes that if the risk of disease in the exposed
group is higher than the risk in the unexposed group,
the difference can be attributed to the exposure.
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37. Measuring of Risk
Measures of association: Attributable proportion
The attributable proportion is the amount of the
disease in the exposed group attributable to the
exposure.
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39. Measuring of Risk
Measures of association: Attributable proportion
Attributable proportion=
Risk for exposed group – risk for unexposed group
x 100%
Risk for exposed group
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40. Measuring of Risk
Measures of association: Attributable proportion
Example
A study of smoking and lung cancer, the lung cancer
mortality rate among nonsmokers was 0.07 per 1,000
persons per year. The lung cancer mortality rate
among who smoked 1-14 cigarettes per day was 0.57
lung cancer deaths per 1,000 persons per year.
Calculate the attributable proportion?
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41. Measuring of Risk
Measures of association: Attributable proportion
Attributable proportion = (0.57 – 0.07) / 0.57X
100%= 87.7%
About 88% of the lung cancer among smokers of 1-14
cigarettes per day might be attributable to their
smoking.
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42. • Relative risk and odds ratio are important as measures of the
strength of association
– Important for deriving causal inference.
• Attributable risk is a measure of how much disease risk is attributed
to a certain exposure
– Useful in determining how much disease can be prevented.
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43. Relative risk is valuable in etiologic studies of disease.
Attributable risk is useful for Public Health guidelines and
planning.
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