Case-control
When We Use Case-control?
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Exposure data are expensive or difficult to obtain
Disease is rare
Chronic diseases (has a long induction and latent period)
Little known about the disease
The underlying population is dynamic
Purpose of control
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Refers to the exposure distribution in the source population or base population
Strength of case-control design
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Most efficient design for studying rare diseases
Most efficient design to study long latent diseases
Quick – no need to follow up
Can evaluate several etiologic factors for a single disease
Limitations
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Inefficient for studying rare exposures
Bias due to sampling two population
Inability to establish correct temporal relation between E AND O sometimes
Cannot compute incidence of diseases (unless design is population-based)
Susceptible to selection and information bias
Sampling of controls: Survivors sampling
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Controls are samples from those who survived in the follow up
At the end of the study
Sampling of controls: Base sampling
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Controls are sampled from the population at risk when follow-up begins
Set our controls at the beginning of the study from the at-risk population
Sampling of controls: Risk set sampling
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Controls are sampled from the population at risk as cases are diagnosed
Once a case appears we set control
Nested case-control design
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Conducted within cohorts of exposed and unexposed
Controls is selected each time a case develops
o Cases and controls are matched on in calendar time and length of follow-up
o Control who selected early in the study can become a case
Case-cohort design
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Controls are randomly selected from defined cohort at the beginning of the study
o Equal chance of being included in the study as a control regardless the person-time
or the person develop the disease of the study
Advantage: controls are not individually matched to each case  possible to study different
diseases in the same case-cohort study
Ratio of controls to cases
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To increase the power of the study to detect an association we increase the size of the
control group
4 controls for each case