Measures of Impact Dublin June 2006. Measures of Impact You want to reduce deaths from road traffic accidents Most impact for least cost Cohort study.

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Presentation transcript:

Measures of Impact Dublin June 2006

Measures of Impact You want to reduce deaths from road traffic accidents Most impact for least cost Cohort study to examine causes for deaths from traffic accidents

Measures of Impact Measures of association providing information about absolute effects of exposure Reflect apparent contribution of an exposure to the frequency of disease Two concepts -attributable risk among exposed -attributable risk among population

1. Impact measures among exposed 2. Impact measures in the population

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)

Attributable Risk I exposed – I unexposed I = Incidence

Dead Not dead Risk RD Fast Slow AR: Fast driving

Dead Not dead Risk RD Fast Slow AR: Fast driving

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

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

Attributable fraction (AF) 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 risk percent (AR%) -Etiologic fraction )

Attributable fraction %

Two points to note 1)I exposed – I unexposed Iexposed = RD/ I exposed 2) I exposed – I unexposed I exposed = 1 – I unexposed /I exposed = 1 – 1/RR

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

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

AR & AF in Case-Control Studies No direct risk estimates in case-control study Direct measure of AR (risk difference) and AF not possible but if Odds Ratio approximates relative risk, then Oui, if you have a good control group... AF = I – I/OR

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

Prevented fraction

PF: Vaccine efficacy

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

1. Impact measures among exposed 2. Impact measures in the population

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)

Population Attributable Risk

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

Population Attributable Fraction

Dead Not dead Risk Fast Slow PAF: Fast driving

Dead Not dead Risk Drunk Not d PAF: Drunk driving

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

PAF in case control studies Another way of expressing PAF PAF = (I – 1/RR) x Pc = AF x Pc where Pc = % cases exposed Case-control study assuming OR = RR PAF = (I-1/OR) x P c

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

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

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 Ryanair or Easyjet (ignoring cost!)? "