Cohort Studies

Question 1

What are cohort studies?

Answer 1

• Observational studies allowing the researcher to passively observe natural events occurring in naturally exposed and unexposed (comparison) groups.

Question 2

What are the two versions of a cohort study?

Answer 2

• Group-allocation based on exposure status

- Group membership because of something in common

Question 3

What are some other names for cohort studies?

Answer 3

• Incidence studies
• Follow-up studies
• Longitudinal studies

Question 4

What is commonly generated from cohort studies?

Answer 4

Risk of disease/outcome.

Risk Ratio/Relative Risk (RR) as a measure of association

Question 5

What are some reasons for selecting a cohort study?

Answer 5

• Unable to force group allocation
• Limited resources
• The exposure of interest is rare in occurrence and little is known about its associations/outcomes (v1)
• More interested in incidence rates or risks for outcome interest (more than effects of interventions)

Question 6

What are the 3 fashions/perspectives that a cohort study can be conducted in?

Answer 6

• Prospective
• Retrospective
• Ambidirectional

Question 7

Prospective Cohort Studies

Answer 7

• Exposure group is selected on the basis of a past or current exposure and both groups followed into future to assess for outcomes of interest and then compared

Question 8

Retrospective (Historical) Cohort Studies

Answer 8

• At the start of the study both exposure and outcome of interest have already occurred, but groups allocated based on past history of exposure.
• Retrospectively start at time of exposure and follow forward to the point of outcome occurrence (known) in the present
• exposure still has to happen before outcome of interest and group allocation is based on exposure status

Question 9

Ambidirectional Cohort Studies

Answer 9

• Uses retrospective design to assess past differences (up to present, but adds future data collected on additional outcomes prospectively from start of study.
• looking for outcomes in the past and into the future

Question 10

What are the types of cohorts that refer to a group with something in common?

Answer 10

• Birth Cohort
• Inception Cohort
• Exposure Cohort

Question 11

Birth Cohort

Answer 11

• Individuals assembled based on being born in a geographic region in a given period of time

Question 12

Inception Cohort

Answer 12

• Individuals assembled at a given point based on some common factor
• Where people live, work, or something they have in common
• Useful for single-group assessments for incidence rate determination

Question 13

Exposure Cohort

Answer 13

• Individuals assembled based on some common exposure

- frequency connected to environmental or other one-time events

Question 14

Fixed Cohort

Answer 14

A cohort which can’t gain members but can have loss-to-follow-ups

Question 15

Closed Cohort

Answer 15

A fixed cohort with no loss-to-follow-ups

Question 16

Open (or Dynamic) Cohort

Answer 16

• Cohort with new additions and some loss-to-follow-ups

- Can increase or decrease over time, as people emigrate or immigrate in and out of cohort (population) being studied

Question 17

How do you select an unexposed study population?

Answer 17

Make the groups as close as possible

Question 18

What 3 sources can the unexposed group come from?

Answer 18

• Internal
• General Population
• Comparison Cohort

Question 19

Internal Source

Answer 19

Patients from the same cohort, who are unexposed

• best, if feasible

Question 20

General Population Group

Answer 20

• used as a second choice when the internal is not realistically possible

(E.g. Everyone is exposed)

Question 21

Comparison Cohort

Answer 21

• Least acceptable group

- simply attempt to match groups as close as possible on numerous personal characteristics

Question 22

What are the strengths of cohort study designs?

Answer 22

• good for assessing multiple outcomes of one exposure
• Useful when exposure are rare
• Useful in calculating Risks and RR
• Less expensive than interventional
• Good when ethical issue limit use of interventional
• Good for long Induction/Latent periods (retro)
• Able to represent temporality (Pro)

Question 23

What are some advantages of Prospective Cohort Studies?

Answer 23

• Can obtain a greater amount of study-important information from patients
• Follow up or tracking patients may be easier
• Better at giving answer to temporality
• May look at multiple outcomes from a (supposed) single exposure
• Can calculate incidence & incidence rates

Question 24

Disadvantages of Prospective Cohort Studies

Answer 24

• Time, Expense, & Lost-to-follow-up
• Not efficient for rare diseases
• Not suited for long induction/latency conditions
• Exposure may change over time

Question 25

Advantages of Retrospective Cohort Studies

Answer 25

- Best for long induction/latency conditions - Able to study rare exposures - Useful if the data already exists - Saves time and money compared to prospective

Question 26

Disadvantages of Retrospective

Answer 26

- Requires access to charts, databases, employment records - information may not factor in or control for exposures to harmful elements during study period or over time - patients may not be available for interview if contact necessary for missing or incomplete data - Exposure may be changed over time

Question 27

Matching

Answer 27

A way to make groups as equal as possible on known/potential confounders

Question 28

Key biases with cohort studies

Answer 28

- Healthy worker effect | - Selection bias