Cohort Studies

Question 1

What are person-years?

Answer 1

The sum of the total time of everybody followed up in a study

Question 2

What are the advantages of cohort studies? (3)

Answer 2

1) Can study exposures and personal characteristics routinely collected 2) Can obtain more detailed information on outcomes and exposures 3) Can collect additional data on potential confounding factors

Question 3

What are the different calculations made if you compare results from a cohort study with an internal population vs an external reference population?

Answer 3

With an internal population- calculate an incidence rate ratio
With an external reference population- calculate an SMR

Question 4

What types of bias are cohort studies subject to?

Answer 4

Selection bias- Healthy worker effect

Survivor bias

Question 5

What is the healthy worker effect?

Answer 5

People in employment are more likely to be healthy and therefore occupational cohorts may skew results

Question 6

Why are you limited when comparing cohorts to an external reference population?

Answer 6

• Often data from an external reference population is limited: there is usually no incidence data
• Study and reference population may not be comparable leading to selection bias

Question 7

When are cohort studies better than case control studies?

Answer 7

• At studying a range of different outcomes
• At studying a rare exposure
• At establishing that exposure precedes outcomes

Question 8

What is a disadvantage of cohort studies?

Answer 8

• They are large and resource intensive
• It takes a long time so results take a long time
• Rigorous definitions of outcomes and exposures
• Not good for rare outcomes

Question 9

Cohort studies see how a group of people’s ………. affect their ………

Answer 9

Cohort studies see how a group of people’s EXPOSURES affect their outcomes