case control 1

7/28/2019 Case Control 1

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Case control

Monday, September 26, 2011


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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%

case control 1

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