1. Class session 13 Case-control studies
Epidemiology 503

2. Hierarchy of Study Designs
Generate Hypotheses
Establish Causality
Good place to start investigating a research question
Ecologic Studies
Cross Sectional Studies
Case Control Studies
Cohort Studies
Randomized Trials
Gold standard for establishing causality

3. Randomized Trials and Cohort Studies
Study Population
Randomization or Self-Selected Exposure
Exposed to factor
Not exposed to factor
Event
No event
Source Population

4. Limitations: RCTs and Cohort Studies
Expensive Lengthy time of follow-up Inefficient for studying rare diseases

5. Case-Control Study Design
Cases
Controls
Disease
No Disease
Exposed
Unexposed
Exposed
Unexposed
Controls should represent exposure in the population that gave rise to the cases

6. Selection of Cases and Controls
Case Definition?
Prevalent or incident?
Where from?
Hospitals
Disease registries
Community
Controls
Where from?
General population controls
Friend controls
Hospital controls
Matching?
For selection of controls, weigh pros and cons: ease of identification vs ability to draw from same source population as cases

7. Measures of Association: CCS
Can we get incidence measures from a case-control study?
Cases
Controls
TOTAL
Exposed a b
a+b No c d
c+d
Can’t calculate risk ratio or rate ratio in a case-cohort study
In cohort study, risk or rate ratio is: Incidence in exposed (a/(a+b))/incidence in unexposed (c/(c+d))
Ratio of disease to no disease is set by investigator

8. Odds Ratio: CCS OR = ad / bc Cases Controls Exposed a b No c d
Odds of exposure among cases = a/c
Odds of exposure among controls = b/d
Cases
Controls
Exposed a b No c d
OR = ad / bc
Interpretation (OR=1.9)
In study, the odds of exposure were1.9 times greater (or 90% greater) among those with disease (cases) than among those without disease (controls).

9. Cohort vs. Case Control Cohort Study Case Control Study
Research Question
Are persons exposed more likely to get disease vs. unexposed?
Are persons with disease more likely to be exposed vs. persons without disease?
Selection of Subjects
With and without exposure of interest.
With and without disease of interest.
Interpretation of OR
Those exposed to XYZ were 2.5 times more likely to develop disease vs. those not exposed to XYZ.
Those with disease were 2.5 times more likely to be exposed to XYZ than those without disease.
Strengths
Good for rare exposures
Can calculate incidence
Can study multiple outcomes
Good for rare diseases
Good for long latency periods
Can study multiple exposures
Limitations
Inefficient for rare diseases
Expensive
Cannot calculate incidence
Inefficient for rare exposures