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!)? "