Measures of Impact

Lazareto, Menorca, SpainSept 2006

L Petersen 1999, T Grein 2000-2004, M Valenciano 2005-2006

• 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

RR = 5.0

0.000000500.00000010

0.500.10

% exposed to risk factor?

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

Attributable risk (AR)

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 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)

Attributable Risk Percent (AR%)

Incidence

Exposed Unexposed

%exposed

unexposedexposed

I

I - I100 x

RR

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

AR%: Fast driving

Dead Not dead Risk AR%

Drunk 45 255 300 0.150

Not d. 135 9565 9700 0.014

180 9820 10000

AR%: Drunk driving

0.150- 0.014 0.150

= 91%

No direct estimate of risk in case

control studies.

Cannot compute impact measures

hee, hee....

AR & AR% in Case-Control Studies

• No risk estimates in case-control study- Attributable risk (risk difference) - and AR percent

calculation not possible

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

100 x OR

1 -OR AR%

Yes, if you have a good control

group...

Prevented fraction

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

Prevented fraction

RR -1

I

I -I PF

unexposed

exposed unexposed

Incidence

Unexposed Unexposed

RR -1

I

I -I

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

I

I -I

unexposed

exposed unexposed

Expected number of cases among vaccinated if unvaccinated

519 1.72 x 1,000

301,545

Observed number of cases 150

Estimated number of cases prevented 369 (72%)

Population attributable risk

• 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

Risk

Population Unexposed

unexposed population I -I

• 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)

100 x I

I - I PAR%

population

unexposedpopulation

Population Attributable Risk Percent (PAR%)

100 x 1 1)-(RR P

1) -(RR P PAR%

100 x I

I - I PAR%

population

unexposedpopulation

where P = proportion population exposed

Dead Not dead Risk

Fast 100 1900 2000 0.050

Slow 80 7920 8000 0.010

180 9820 10000 0.018

PAR%: 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

PAR%: 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% presumably due to fast driving

- 22% presumably due to drunk driving

PAR% in USA and Italy for cervical cancer,

various risk factors

Source, Parazzini et al. 1990

PAR% in case control studies

• Cohort study

• Case-control study

If % controls exposed % population exposed

where P = % population exposed

where Pcon = % controls exposed

100 x 1 1)-(RR P

1) -(RR P PAR%

100 x 1 1)-(OR P

1) -(OR P PAR%con

con

Summary

• Among exposed - Attributable risk- Attributable risk percent- Prevented fraction

• In the population- Population Attributable Risk- Population Attributable Risk percent

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 Georgian Airlines or Lufthansa?"

100 x 1 1)-(RR P

1) -(RR P PAR%

I

I PAR%population

unexposedpopulation I -

I population = Pe(Iexposed) + (1-Pe)Iunexposed

unexposedexposed

unexposedunexposedexposed

Pe)I-(1 ) (I Pe

I - Pe)I-(1 ) (I Pe

Divide numerator and denominator by Iunexposed