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Measures of Impact Lazareto, Menorca, Spain Sept 2006 L Petersen 1999, T Grein , M Valenciano

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You are in charge of health prevention Want to reduce automobile-related deaths Limited budget but want to have the greatest impact on reducing deaths Cohort study to examine causes for automobile-related deaths

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RR = % exposed to risk factor?

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Measures of Impact Measures of association providing information about absolute effects of exposure Reflect apparent contribution of an exposure to the frequency of disease Three concepts -Attributable risk among exposed -Prevented fraction among exposed -Population attributable risk

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Attributable risk (AR)

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Attributable Risk (AR) Quantifies disease burden in exposed group attributable to exposure Provides answers to -what is the risk attributed to the exposure? -what is the excess risk due to the exposure? Calculated as risk difference (RD)

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Attributable Risk I exposed – I unexposed I = Incidence

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Dead Not dead Risk RD Fast Slow AR: Fast driving

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Dead Not dead Risk RD Fast Slow AR: Fast driving

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Drunk Not d AR: Drunk driving Dead Not dead Risk RD

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Drunk Not d AR: Drunk driving Dead Not dead Risk RD

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Attributable Risk Percent (AR%) Etiologic fraction Attributable risk expressed as a percentage of risk in exposed What is the proportion of disease among the exposed which … -can be attributed to the exposure? -could be avoided by eliminating the exposure? Synonyms -Attributable proportion -Attributable fraction -Etiologic fraction (EF)

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Attributable Risk Percent (AR%) %

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Dead Not dead Risk AR% Fast = 80% Slow AR%: Fast driving

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Dead Not dead Risk AR% Drunk Not d AR%: Drunk driving = 91%

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No direct estimate of risk in case control studies. Cannot compute impact measures hee, hee....

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AR & AR% in Case-Control Studies No risk estimates in case-control study -Attributable risk (risk difference) -and AR percent calculation not possible

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AR & AR% in Case-Control Studies No direct risk estimates in case-control study -AR (risk difference) and AR% calculation not possible If odds ratio approximates relative risk, then Yes, if you have a good control group...

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Prevented fraction

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Prevented Fraction (PF) For exposures associated with decreased risk If relative risk <1 -exposure is protective Proportion of potential cases -which would have occurred if the exposure had been absent -prevented by the exposure

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Prevented fraction Unexposed

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PF: Vaccine efficacy

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Expected number of cases among vaccinated if unvaccinated Observed number of cases 150 Estimated number of cases prevented 369 (72%)

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Population attributable risk

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Excess risk of disease in total population attributable to exposure Reduction in risk achieved if population entirely unexposed Helps determining exposures relevant to public health in community Population Attributable Risk (PAR)

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Population Attributable Risk

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Proportion of cases in the population attributable to the exposure PAR expressed as a percentage of total risk in population Population Attributable Risk Percent (PAR%) (Population Attributable Fraction)

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Population Attributable Risk Percent (PAR%) where P = proportion population exposed

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Dead Not dead Risk Fast Slow PAR%: Fast driving

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Dead Not dead Risk Drunk Not d PAR%: Drunk driving

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Conclude Driving related deaths in population -44% presumably due to fast driving -22% presumably due to drunk driving

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PAR% in USA and Italy for cervical cancer, various risk factors Source, Parazzini et al. 1990

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PAR% in case control studies Cohort study Case-control study If % controls exposed % population exposed where P = % population exposed where P con = % controls exposed

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Summary Among exposed -Attributable risk -Attributable risk percent -Prevented fraction In the population -Population Attributable Risk -Population Attributable Risk percent

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Summary Where will you put your money to have the greatest impact on reducing automobile-related deaths?

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What is the Appropriate Measure? "Control of prostitution will prevent most heterosexual HIV transmission! "He got lung cancer… But he probably would have gotten lung cancer anyway even if he didn't smoke. "Should I fly Georgian Airlines or Lufthansa?"

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I population = Pe(I exposed ) + (1-Pe)I unexposed Divide numerator and denominator by Iunexposed

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