measuring of risk

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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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• 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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Epidemiology determine the relationship or association between a given exposure and

frequency of disease in population.

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Epidemiology does not determine the cause of a disease in a given individual.

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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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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


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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.


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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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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


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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RR is mostly used in Cohort StudiesCan be Used also in Cross sectional as

well as Clinical trails.

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Relative risk calculator

http://www.medcalc.org/calc/relative_risk.php

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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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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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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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Disease+ Disease- Total

Exposed 100 1900 2000

Not Exposed 80 7920 8000

Total 180 9820 10000


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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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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Odds Ratio calculator

http://www.medcalc.org/calc/odds_ratio.php

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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.


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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.


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The attributable proportion is the amount of the disease in the exposed group attributable to the

exposure.


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Attributable proportion=

Risk for exposed group – risk for unexposed group x 100%

Risk for exposed group


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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?


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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.


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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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Practical Exercise

http://www.uoguelph.ca/numeracy/lofiles/OR%20and%20RR.swf

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THANK YOU

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