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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
Incidence
Exposed Unexposed
Iexposed – Iunexposed
I = Incidence
Dead Not dead Risk RD
Fast 100 1900 2000 0.05
0.04
Slow 80 7920 8000 0.01
180 9820 10000
AR: Fast driving
Dead Not dead Risk RD
Fast 100 1900 2000 0.05
0.04
Slow 80 7920 8000 0.01
180 9820 10000
AR: Fast driving
Drunk 45 255 300 0.150
0.136
Not d. 135 9565 9700 0.014
180 9820 10000
AR: Drunk driving
Dead Not dead Risk RD
Drunk 45 255 300 0.150
0.136
Not d. 135 9565 9700 0.014
180 9820 10000
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
Incidence
Exposed Unexposed
%exposed
unexposedexposed
I
I - I
Attributable fraction
Two points to note
1) Iexposed – Iunexposed Iexposed
= RD/ Iexposed
2) Iexposed – Iunexposed
Iexposed
= 1 – Iunexposed/Iexposed
= 1 – 1/RR
Dead Not dead Risk AR%
Fast 100 1900 2000 0.0500.050- 0.010 0.050
= 80%Slow 80 7920 8000 0.010
180 9820 10000
AF: Fast driving
Dead Not dead Risk AR%
Drunk 45 255 300 0.150
Not d. 135 9565 9700 0.014
180 9820 10000
AF: Drunk driving
0.150- 0.014 0.150
= 91%
AR & AF in Case-Control Studies
• No direct risk estimates in case-control studyDirect 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
RR -1
I
I -I PF
unexposed
exposed unexposed
PF: Vaccine efficacy
Pop. Cases Cases/1000 RR
Vaccinated 301,545 150 0.49 0.28
Unvaccinated 298,655 515 1.72 Ref.
Total 600,200 665 1.11
0.72 0.28 - 1
0.72 1.72
0.49 - 1.72 PF
Expected number of cases amongvaccinated if unvaccinated
519 1.72 x 1,000
301,545
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 (PAR)
unexposedpopulation I - IPAR
Population Attributable Risk
Risk
Population Unexposed
unexposed population I -I
• 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
100 x I
I - I PAR%
population
unexposedpopulation
Dead Not dead Risk
Fast 100 1900 2000 0.050
Slow 80 7920 8000 0.010
180 9820 10000 0.018
PAF: Fast driving
44% 100 x 0.018
0.010 - 0.018 PAR%
0.008 0.010 - 0.018 PAR
Dead Not dead Risk
Drunk 45 255 300 0.150
Not d. 135 9565 9700 0.014
180 9820 10000 0.018
PAF: Drunk driving
22% 100 x 0.018
0.014 - 0.018 PAR%
0.004 0.014 - 0.018 PAR
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 Pcwhere Pc = % cases exposed
• Case-control studyassuming OR = RR
PAF = (I-1/OR) x Pc
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!)? "