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RELATIVE RISK

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Measures of association

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A measure of association quantifies the relationship between exposure and outcome between two groups

Outlines

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  • Relative risk = risk ratio

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  • Odds ratio

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Relative risk

HOW MUCH LIKELY ? Exposed person develops new outcome

relative to

Unexposed person to develop new outcome

More..

 

Start with exposure check for outcome

exposure

outcome

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Cohort study recruits 100 people exposed to hypertension and 100 people had not been exposed to hypertensin and follow them up for 1 year to assess the probability to develop heart attack among these two group. 9 of 100 exposed group develop heart attack compered with 3 of 100 of unexposed group.

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Calculate the relative risk

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Do Not develop

Heart

Attack

develop

Heart Attack

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91

9

100 with

hypertension

97

3

100 without

hypertension

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Interpretation of relative risk

If RR =1 no increases risk no association

The incidence in the exposed group is the same as the incidence in the unexposed

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If RR>1 increases risk positive association

The incidence in the exposed group is greater than the incidence in the unexposed

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If RR<1 decresed risk negative association

The incidence in the exposed group is lower than the incidence in the unexposed

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Another example

In an outbreak of varicella (chickenpox) in Oregon in 2002, varicella was diagnosed in 18 of 152 vaccinated children compared with 3 of 7 unvaccinated children.

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Calculate the relative risk

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Interpretate the result

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80%

94%

Do Not had

Varicella

Had

Varicella

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134

18

152 vaccinated

4

3

7 unvaccinated

 

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Relative risk is useful tool to assess the association in cohort studies

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Relative risk needs incidence , probability of new outcome occurring during period

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What if incidence can't be collected ??

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Case control study:

  • start with outcome of interest , and lock back te see what might have caused it.

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- Helpful in rare disease or anoutome

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Start with exposure, check for the outcome

Initial data: exposure

Data of interest: weather or not the patient develop the outcome

Good for: rare outcome

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Statistical analysis: relative risk

Start with outcome, check for the exposure

Initial data: outcome

Data of interest: weather or not the patient have been exposed

Good for: rare exposure

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Statical analysis : odds ratio

Cohort study vs case-control study

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ODDS RATIO

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