Measures of Impact Lazareto, Menorca, Spain Sept 2006

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Measures of ImpactLazareto, Menorca,
SpainSept 2006

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

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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 5.0
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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
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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
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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
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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
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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
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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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AR Fast driving
Dead Not dead Risk
AR
Fast 100 1900 2000 0.050
0.050- 0.010 0.050 80
Slow 80 7920 8000 0.010
180 9820 10000
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AR Drunk driving
Dead Not dead Risk
AR
Drunk 45 255 300
0.150
0.150- 0.014 0.150 91
Not d. 135 9565 9700
0.014
180 9820 10000
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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
Yes, if you have a good control group...

• No direct risk estimates
  in case-control study
• AR (risk difference)
• and AR calculation not possible
• If odds ratio
  approximates relative risk, then

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Prevented fraction
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Prevented Fraction (PF)

• For exposures associated with decreased risk
• If relative risk lt1
• 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
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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Population Attributable Risk (PAR)

• 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

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Population Attributable Risk
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Population Attributable Risk Percent (PAR)
(Population Attributable Fraction)

• Proportion of cases in the population
  attributable to the exposure
• PAR expressed
  as a percentage of total
  risk in population

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Population Attributable Risk Percent (PAR)
where P proportion population exposed
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PAR Fast driving
Dead Not dead Risk
Fast 100 1900 2000
0.050
Slow 80 7920 8000
0.010
180 9820 10000
0.018
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PAR Drunk driving
Dead Not dead Risk
Drunk 45 255 300
0.150
Not d. 135 9565 9700
0.014
180 9820 10000
0.018
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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 Pcon 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(Iexposed) (1-Pe)Iunexposed
Divide numerator and denominator by Iunexposed