Upload
emma-stuart
View
217
Download
4
Tags:
Embed Size (px)
Citation preview
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