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Case control
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A Population
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A Population
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From the source population we select 17 individuals who were exposed to high levels of benzene.
Our study is designed to examine the effect of benzene exposure on the eventual developmentof adult leukemia
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Next we select a random sample of 83 workers who have no occupational exposure to benzene
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Follow-up lasts 20 years
No loss to follow-up!
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20 Years Later
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20 Years Later
After 20-years there are 6 cases in the
exposed6 people x 10-years+11 people x 20-years=280 person-years
6 cases / 280 person-years = 0.021cases/person-year or 21 cases per 1,000person years
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20-years
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Measures of effect in cohort study
IRR = 21/3.2 = 6.57
Workers exposed to benzene were 6.57 times as likely todevelop leukemia after 20-years as workers who were notexposed to benzene
IRD = 21-3.2 = 17.8 cases per 1,000 person years There were an additional 17.8 cases per year for each 1,000
workers exposed to benzene.
Or, if we prevented the exposure in 1,000 people, 17.8 caseswould be avoided annually (assuming a causal relationship exists)
Attributable fraction in the exposed 17.8/21 = 0.852 = 85% of the cases in the exposed are due to
the exposure (assuming a causal relationship)
Or, (6.57-1)/6.57 = 85% attributed to exposure in the exposed
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Case-Control study in the same population
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Cases
CaseDefini*on
Partofthedefini*onisclassicepidemiology
Person,Place,andTime
Alsoincludediagnos*ctests,clinicalandpathological
examsetc.
Useasmuchinforma*onasisavailable
Sourceofcases
Hospitals,popula*onbased,diseaseregistry
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Now we choose incident cases
Define our cases
Workers in factories who develop
leukemia (or developed leukemia ifretrospective) over the 20-year period
Take a sample of cases (or enroll all cases
if possible)
Case selection
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Controls
Asampleofthesourcepopula*onthatgaveriseto
thecases
Inacohortstudyyouarefollowingthissource
popula*onun*ltheydevelopdisease Inacase-controlstudy,yousamplethispopula*onto
assesstheexposuredistribu*oninthepopula*on
Thewouldcriterion
Thesourcepopula*onisagroupwhosememberswouldendupasacaseinthestudyiftheydevelopedthe
disease
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Controls Hospitalizedcontrols
Mustmakesurethatthecondi*onforwhichtheyarein
thehospitalhasnorela*onshipwiththediseaseor
exposureofinterest
Theillnessinthecontrolgroupshouldhaveasimilarreferralpaerntothediseaseyouarestudying
Popula*onControls
Moregeneralizable
Likelythesamesourcepopula*onthatgaverisetocases
Timeconsuming,lessinteresttopar*cipate,recallbias
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Control selection
The most important criteria: if these non-cases wouldhave become diseasedwould we have identified themas cases?
This ensures that the following goals for controlselection are met:
Controls come from the same source populationas cases
The exposure distribution in the controls shouldmatch the exposure distribution of the entiresource population
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= case
Take random sample of non-cases. Well choose 22 tohave 2:1 ratio of controls to cases
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6 exposed cases5 unexposed cases
3 exposed non-cases18 unexposed non-cases
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Measure of effect in case-control
Odds ratio
OR = odds of exposure
among cases divided by oddsof exposure among controls
Can calculate this with thecross product ratio:
(6*18)/(5*3)
Cases Non-Cases
Exposed 6 3
Unexposed 5 18
7.2
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Effect measures
Odds ratio? yes
Difference measures? no
Attributable fraction in exposed? yes
(7.2-1)/7.2=86%
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