Lecture 19 - Cohort Studies

Question 1

What is the more common type of cohort study?

Answer 1

• observational study where individuals are grouped based from exposure status
• useful for rare exposures
• generates a risk ratio

Question 2

What type of ratio is a cohort study most useful in generating?

Answer 2

-risk ratio

Question 3

What is the type of cohort study that is not based on initial group allocation?

Answer 3

• starts with a “cohort” of people that have something in common
• initial grouping can be with regards to either exposure OR disease status

Question 4

Differentiate between prospective, retrospective, and ambidirectional cohort study designs.

Answer 4

Prospective:
-separate by exposure status and then follow groups to assess for outcomes

Retrospective:

• exposure status and disease status are already known
• groups are separated by exposure status and then assessed looked at for disease status

Ambidirectional:
-uses retrospective design but then follows groups for further data

Question 5

What are examples of different types of cohorts?

Answer 5

Birth cohort:
-commonality of being born in the same region around the same time

Inception cohort:
-common factor such as where people live or work

Exposure cohort:
-common exposure

Question 6

What ways do membership in cohort studies change over time?

Slide 22

Answer 6

Fixed:
-can’t gain members but can lose them to follow up

Closed:
-can’t gain members and does not lose members to follow up

Open (Dynamic):
-can gain and lose members

Question 7

What sources are available to pick unexposed individuals from?

Answer 7

Internal (best):

• from the same “cohort” as exposed but are not exposed themselves
• could also be the least exposed if there is nobody unexposed in the cohort

General population:
-unexposed or least exposed from the general population the exposed were taken from

Comparison (worst):

• match unexposed with exposed based off personal characteristics
• doesn’t control for other variable which may cause disease

Question 8

What are strengths of cohort studies? (6)

Answer 8

• good for assessing multiple outcomes of one exposure
• useful for rare exposures
• useful for producing risk ratios
• less expensive than interventional
• good for long induction/latency periods (retrospective)
• can represent temporality (prospective)

Question 9

What are some weaknesses of cohort studies? (6)

Answer 9

• can’t demonstrate causation
• difficult to control for other exposures or changes in exposure (primarily retrospective)
• not good for long induction/latent periods (prospective)
• can be impacted by unexpected confounders
• can be impacted by biases
• limited by available data

Question 10

What are advantages of prospective cohort studies? (5)

Slide 30

Answer 10

• more control over data collection process
• follow-up is easier
• better answers temporality
• can look at multiple outcomes from a single exposure
• can produce incidence rates

Question 11

What are disadvantages of prospective cohort studies? (6)

Slide 31

Answer 11

• time consuming
• expensive
• lost to follow up
• not useful for rare diseases
• not suitable for long induction/latency periods
• degree of exposure can change (not controlled for)

Question 12

What are advantages of retrospective cohort studies? (4)

Slide 33

Answer 12

• useful for long induction/latency periods
• useful in rare diseases
• useful if data is already collected
• less expensive and time consuming (compared to prospective)

Question 13

What are the disadvantages of a retrospective cohort study? (5)

(Slide 34)

Answer 13

• requires access to data
• resources might not contain all desired information
• no control for other exposures
• can be difficult to follow-up on patients
• exposure may have changed

Question 14

What are common biases with cohort studies?

Slide 36

Answer 14

• healthy worker bias

- selection bias