Cohort Studies

Question 0

Name and explain the two types of cohort study

Answer 0

Prospective (based on current exposure status) - follow up starts immediately or later
Retrospective - collect data from the past

Question 1

Explain how to organise a cohort study

Answer 1

Recruit disease-free individuals
Classify into exposed and unexposed
Follow each group over time (years)
Count how many develop the disease being studied
Calculate the IR for each group then IRR

Question 2

Explain internal and external comparisons

Answer 2

Internal - compare exposed and unexposed in cohort with sub-cohorts within original group (IRR)
External - compare exposed population with a reference population (SMR)

Question 3

List and explain the different types of bias

Answer 3

Selection bias (error due to systematic differences in ways which the two groups were collected) - allocation bias, healthy worker effect
Information bias (error due to systematic misclassification of subjects in the group) - recall bias, publication bias

Question 4

Explain the healthy worker effect

Answer 4

There is bias when a study involving workers/unemployed people is compared to the reference population - an employed person is more likely to be healthier than an unemployed person

Question 5

List some general problems with cohort studies

Answer 5

Usually large and resource intensive –> expensive
Take a long time
Risk of high losses to follow up (survivor bias)
Results take a long time (ethical considerations)
Not good for rare diseases (too few cases)
Difficulty with confounding, especially unknown ones