Cohort Studies

Question 1

What is a cohort study?

Answer 1

Allows for analytic epidemiology (exposures and outcomes) and uses observational methods (observe peoples exposures and what happens to them)

‘Individuals are defined on the bassi of presence or absence of exposure to a suspected risk factor’

Question 2

Describe cohort studies step by step

Answer 2

1. Identify a source population
2. Recruit your sample population (must not have the outcome of interest!)
3. Assess exposure to identify which group participants belong in (exposed or not exposed)
4. Follow up over time
5. Observe whether or not participants develop the outcome
6. Calculate measures of occurrence and measures of association

Question 3

What can we measure using cohort studies?

Answer 3

Incidence proportion: number of people who develop a disease in a specific period / number of people at risk of developing the disease at the start of the period

Incidence rate: number of people who develop a disease in a specific period / number of person-years at risk of developing the disease

Relative risk and risk difference (attributed risk)

Question 4

What might we need to consider carefully at each of the steps of the study?

Answer 4

1 and 2:
- Ideal: random selection independent of exposure status
- Sometimes: selection based on exposure status (must consider appropriate comparison group)
- Can you be sure that the sample population does not already have the outcome?
3.
- Have participants been correctly classified (into exposure and comparison)?
4.
- Have participants changed exposure status over time?
- Has everyone been followed up over the entire study?
- How long to participants need to be followed up?
5.
- Has the outcome status been correctly classified?

Question 5

What is the healthy worker effect?

Answer 5

An example of selection bias. Could be a study on injuries at work, however people who work are generally fitter than people who don’t. So if the comparison is the general population, then generally workers will be healthier.

Question 6

What are the strengths of cohort studies?

Answer 6

• Determine temporal sequence between exposure and outcome
• Can examine multiple outcomes form an exposure
• Can calculate incidence (and therefor relative risk and risk difference/attributable risk)
• Good for studying rare exposures

Question 7

What are the limitations of cohort studies?

Answer 7

• Loss to follow up (can lead to bias if related to the exposure and the outcome)
• Potential for misclassification of exposures/outcomes
• Generally not good for studying rare outcomes
• Time consuming
• Can be expensive

Question 8

What is the difference between perspective cohort studies and historical cohort studies??

Answer 8

The difference is the starting point.
- Perspective cohort studies: start at the exposure and work through the time being followed up and the outcomes
- Historical cohort studies: start at the end of the process with outcomes and work backwards using existing data. So they reconstruct the follow up period in the past

Question 9

What are the pros and cons of historical cohort studies?

Answer 9

Pros:
- Less time consuming compared with protective cohort studies
- Good for outcomes that take a long time to develop
- Less expensive

Cons:
- Use existing data (collected for other reasons) so the quality??
- May not know about all relevant factors
- Bias??