1. EPID 503 – Class 12
Cohort Study Design

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

3. Recall – Randomized Trials
Study Population
RANDOMIZATION
Exposed to factor
Not exposed to factor
Event
No event
Source Population

4. Thought Question
Why do we not always do RCTs if they are one of the best designs?

5. Randomized Trials Not Always Appropriate

6. Cohort Study Design Represents Another Way
Key features:
Start with exposed and unexposed and observe over time to see who develops disease
Participants must be “at risk” of developing outcome
Study Population
Self Selection
Not exposed
Exposed
Event
No event
Event
No event

7. What are the Strengths of This Design?
Study Population
Self Selection
Not exposed
Exposed
Event
No event
Event
No event

8. What are the Strengths of This Design?
Can estimate incidence
Know that exposure preceded disease
Can study rare exposures
Can study multiple outcomes
Study Population
Self Selection
Not exposed
Exposed
Event
No event
Event
No event

9. What are the Strengths of This Design?
Study Population
Gold Standard of Observational Designs
Self Selection
Not exposed
Exposed
Event
No event
Event
No event

10. What are the Limitations of This Design?
People self-select exposures so exposed and unexposed may differ with respect to important characteristics
Cannot study rare diseases
Prospective design is expensive
Prospective design can require long follow-up
Study Population
Self Selection
Not exposed
Exposed
Event
No event
Event
No event

11. How to Get Around Long-Follow Up?

12. How do we analyze and summarize data from a cohort?

13. Disease Yes No Total a b a + b c d c + d a + c b + d a + b+ c+ d
Exposure
What measure can we calculate here?

14. Disease Yes No Total a b a + b c d c + d a + c b + d a + b+ c+ d
Exposure
Cumulative Incidence exp = a/(a+b)
= Risk Ratio
Cumulative Incidence no exp = c/(c+d)

15. Disease Yes Person-Time a PTexp No c PTunexp Total a + c Total PT
Exposure
What measure can we calculate here?

16. Disease Yes Person-Time a PTexp No c PTunexp Total a + c Total PT
Exposure
Incidence Rate exp = a/PTexp
= Rate Ratio
Incidence Rate no exp = c/PTunexp

17. Example Ratio Interpretation
Risk Ratio (or Rate) Ratio = 1.1
Among the population studied during the time frame of interest, the risk (or rate) of disease in exposed persons was 1.1 times that of unexposed persons.
Among the population studied during the time frame of interest, the risk (or rate) of disease in exposed persons was 10% higher than that in unexposed persons.

18. Confidence Intervals
Used to assess the statistical significance of an estimate

19. Interpreting Confidence Intervals
Offers information about our confidence in the point estimate
The wider they are the less confident we are
Confidence intervals get more narrow the greater the sample size (holding all else equal)
Based on hypothetical random replication of the data