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1
Measuring of Risk
Prepared By:Dr.Anees AlSaadiCMT-P R4December 2013
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Measuring of RiskIntroduction:
Link / Relation Disease Exposure
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Measuring of RiskIntroduction:
• 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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Measuring of RiskIntroduction:
Epidemiology determine the relationship or association between a given exposure and
frequency of disease in population.
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Measuring of RiskIntroduction:
Epidemiology does not determine the cause of a disease in a given individual.
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Measuring of RiskIntroduction:
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Measuring of Risk
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Measuring of RiskMeasures 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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Examples of Measure of Association:Risk Ratio (Relative Risk).
Odds Ratio.
Attributable proportion.
Measuring of RiskMeasures of association:
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• Called also relative risk.
• Compares the risk of a health event among one group with the risk among another group.
Measuring of RiskMeasures of association: Risk Ratio
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Risk Ratio (RR)=
Risk of disease in group of primary interest
Risk of disease in comparison group
Measuring of RiskMeasures of association: Risk Ratio(RR)
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Measuring of RiskMeasures of association: Risk Ratio(RR)
Indicates an increase risk for the group in the numerator usually the exposed group.
RR of more than 1.0
Indicates identical risk among the two groups.
RR of 1.0
Indicates a decrease risk for the exposed group.
Indicating that perhaps exposure actually protects against disease occurrence.
RR less than 1.0
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Ill Well Total
Exposed a b a + b
Unexposed c d c + d
Total a+c b+d a+ b+ c+ d
Measuring of RiskMeasures of association: Risk Ratio(RR)
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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.
Measuring of RiskMeasures of association: Risk Ratio(RR)
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Measuring of RiskMeasures 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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Measuring of RiskMeasures 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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Varicella + Varicella - Total
Vaccinated 18 134 152
Unvaccinated 3 4 7
Total 21 138 159
Measuring of RiskMeasures of association: Risk Ratio(RR)
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 as were unvaccinated children.
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Measuring of RiskMeasures of association: Risk Ratio(RR)
RR is mostly used in Cohort StudiesCan be Used also in Cross sectional as
well as Clinical trails.
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Interpretation
Outcome/ Disease/
Conditions/ event
(RR Value) as likely to
develop
Exposure, Risk factor
Patients/ Subjects /
Individuals.
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Measuring of RiskMeasures 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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Measuring of RiskMeasures 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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Measuring of RiskMeasures 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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Measuring of RiskMeasures of association: Odds Ratio(OD)
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Measuring of RiskMeasures of association: Odds Ratio(OD)
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Disease+ Disease- Total
Exposed 100 1900 2000
Not Exposed 80 7920 8000
Total 180 9820 10000
Measuring of RiskMeasures of association: Odds Ratio(OD)
OD= 100X7920 / 1900X80 = 5.2
Example
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Interpretation
Controls without
the exposure
Odds Ratio Value
Case / disease/ Event …
Exposed Subjects/
Individuals ..
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Measuring of RiskMeasures 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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CasesLung Cancer
ControlsWithout Lung Cancer
Total
Daily Smokers 33 55 88
Nonsmokers 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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Measuring of RiskMeasures of association: Odds Ratio(OD)
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Any relation between RR and OR ….?
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Any relation between RR and OR ….?
Relative Risk • Needs incidence of the
disease.• Usually in Prospective, cross
sectional and clinical trail Studies.
• demonstrates temporality.
Odds Ratio• Incidence not for its calculation.• Retrospective studies and in
cross sectional. • Good estimate for RR in case of
low frequency disease. • OR can overestimate risk, in
rare disease.
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Measuring of RiskMeasures 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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The calculation of this measure assumes that occurrence of disease in the unexposed group represent the baseline or expected risk for that
disease.
Measuring of RiskMeasures of association: Attributable proportion
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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.
Measuring of RiskMeasures of association: Attributable proportion
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The attributable proportion is the amount of the disease in the exposed group attributable to the
exposure.
Measuring of RiskMeasures of association: Attributable proportion
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Measuring of RiskMeasures of association: Attributable proportion
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Attributable proportion=
Risk for exposed group – risk for unexposed group x 100%
Risk for exposed group
Measuring of RiskMeasures of association: Attributable proportion
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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?
Measuring of RiskMeasures of association: Attributable proportion
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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.
Measuring of RiskMeasures of association: Attributable proportion
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• 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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Relative risk is valuable in etiologic studies of disease.
Attributable risk is useful for Public Health guidelines and planning.
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Practical Exercise
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THANK YOU