1. Study design IV: Cohort Studies
Principles of design and threats to validity
Brent Hagel, PhD

2. Cohort study
A Cohort – ‘designated group of persons’ with characteristic of interest (eg disease, class of medical students)
Descriptive OR analytical
Prospective OR retrospective

3. Cohort Study
Study groups defined in terms of exposure and followed to determine frequency of outcome
Prospective or concurrent cohort Study
Retrospective or historical Cohort study
Studies can have both prospective and retrospective components
Exposure
Disease ? ?
Study Begins
Exposure
Disease ? ?
Study Begins
From Hennekens & Buring. Epidemiology in Medicine. Little Brown and Company, 1987; P. 24

4. Retrospective cohort study
Identification of study groups based on exposure
Some or all of the subjects may have developed outcome
Look at frequency of outcome in exposed and unexposed
Example question:
Among factory workers, how many exposed to asbestos develop lung cancer compared with unexposed workers?

5. Among factory workers, how many exposed to asbestos develop lung cancer compared with unexposed workers?
Lung Ca
Asbestos
No lung Ca
Lung Ca
No Asbestos
No lung Ca
Investigator

6. Retrospective cohort study design
Identify cohort with exposure of interest
Collect data about other potential predictor variables
How do you choose predictor variables?
Collect data about subsequent outcome (e.g., medical records)

7. Retrospective cohort study - strengths
Establish that potential cause preceded outcome
Potential to evaluate multiple outcomes
‘Cheap’ and quick – the outcome has already occurred

8. Retrospective cohort study - weaknesses
Little control over cohort (what you see is what you get)
No control over data originally collected
What was collected
How it was collected/recorded
Association not necessarily causation

9. Prospective cohort study
Identify subjects based on exposure
Measurement of characteristics at baseline
Follow-up over time for outcome

10. Among factory workers, how many exposed to asbestos develop lung cancer compared with unexposed workers?
Lung Ca
Asbestos
No lung Ca
Lung Ca
No Asbestos
No lung Ca
Investigator

11. Prospective cohort study - strengths
Useful for assessing incidence
Establish temporal relationship
Reduces recall and recording bias, because have baseline and outcome measurement at specified times
More accurate and complete data collection
Efficient for common outcomes

12. Prospective cohort study - weaknesses
Expensive and inefficient for rare outcomes
Prolonged follow-up may be needed
Labour intensive
Association not necessarily causation

13. Validity – Selection bias
Defn.
Distortion in estimate of effect based on way subjects selected for study
Selection probabilities different for different cells of 2x2 table
e.g., those exposed to asbestos more likely to die and more likely to be lost to follow-up
Lack of association between asbestos and lung cancer
Mitigate:
Ensure high response rates
Sensitivity analysis
Carefully consider direction of bias

14. Validity – Misclassification bias
Defn. – differential vs. non-differential
Differential: knowledge of outcome influences exposure ascertainment or vice-versa
Non-differential: classification errors same for cases and controls or exposed and non-exposed
e.g., Differential: research assistants look harder for lung cancer outcomes among those in asbestos factories than in the unexposed industry or industries
“Creates” an association between asbestos and lung cancer

15. Validity – Misclassification bias (cont)
Mitigate by:
Similar procedures for exposed and unexposed
Blind interviewers to exposure status
Use objective rather than subjective information
Use pre-recorded data if available

16. Validity – Confounding bias
Defn.
Risk factor for outcome
Associated with exposure
Not affected by exposure or disease
e.g., smoking an independent risk factor for lung cancer, and if asbestos workers smoke more than non-asbestos industries, could partially account for association
Mitigate
Matching
Document and control
Restriction

17. Design a Cohort Study . . .
Research Question: “What is the risk of sustaining a head or neck injury among intercollegiate ice hockey players wearing full face shields compared with those wearing half shields?"
You have 30 minutes!

18. Cheat sheet Choice of design
reasons for using a cohort study
reasons against a cohort study
How would you identify and select your exposure groups?
How would you define your outcome variable?
When would you measure outcome?
What other variables would be important to consider in this study?