Classes 22-23 Cohort Studies

Question 1

Cohort studies

Answer 1

Observational, analytical studies allowing researcher to be a passive observer of natural events occurring in naturally-exposed and unexposed (comparison) groups

Question 2

In Cohort studies, group-allocation is based on:

Answer 2

Exposure-status or group membership (something in common)

Question 3

When are Cohort studies useful?

Answer 3

Useful when studying a rare exposure

Question 4

What are other names for Cohort studies?

Answer 4

Incidence studies
Follow-up studies
Longitudinal studies

Question 5

Cohort studies commonly generate the ____ ____ as a measure of association

Answer 5

Risk Ratio (RR)

Question 6

Reasons to select a Cohort design

Answer 6

Unable to “randomize”
Limited resources
Exposure of interest is rare
More interested in incidence rates/predictors of or risks for outcome of interest

Question 7

True or False:

Cohort studies can only be done in a prospective fashion.

Answer 7

False

They can be conducted in a prospective, retrospective, or ambidirectional fasion.

Question 8

Prospective Cohort Studies

Answer 8

Exposure group is selected on the basis of a past or current exposure and both groups followed into the future to assess for outcome(s) of interest, and then compared

Question 9

Retrospective Cohort Studies

Answer 9

At the start of the study, both the exposure and the outcome of interest have occurred
Exposure still has to occur before outcome of interest & group allocation is based on exposure status, not disease status

Question 10

Ambidirectional Cohort Studies

Answer 10

Uses retrospective design to assess past differences but adds all data collected on additional outcomes prospectively from start of study (looking for outcomes in the past and into the future)

Question 11

Birth Cohort

Answer 11

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

Question 12

Inception Cohort

Answer 12

Individuals assembled at a given point based on some common factor (ex. Framingham study and Nurses’ health study)

Question 13

Exposure Cohort

Answer 13

Individuals assembled based on some common exposure (frequently connected to environmental or other one-time event)

Question 14

Fixed Cohort

Answer 14

A cohort (derived from an irrevocable event) 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

A cohort with new additions and some loss-to-follow-ups (can increase or decrease over time)

Question 17

How to select an Exposed study population

Answer 17

The easier part

Allocate subject based on pre-defined criteria of “exposure”

Question 18

How to select an Unexposed study population

Answer 18

Make the groups as close as possible (coming from the same cohort/population (yet not exposed))
Can come from 3 sources: Internal, general population, or comparison cohort

Question 19

Unexposed group from INTERNAL source

Answer 19

Patients from the same “cohort”, yet who are unexposed (most similar)
If there are only levels of exposure, you may have to use the lowest exposure group as comparator

Question 20

Unexposed group from GENERAL POPULATION source

Answer 20

Used as a second choice when the best-possible comparison group (internal) is not realistically possible (e.g., everyone is exposed; or the exposure subjects were drawn from the general population)
Randomly selecting

Question 21

Unexposed group from COMPARISON COHORT source

Answer 21

Least acceptable group
Simply attempt to match groups as close as possible on numerous personal characteristics (can’t control for other potentially harmful exposures in comparison cohort; also causing disease)

Question 22

Strengths of Cohort Studies

Answer 22

Good for assessing multiple outcomes of one exposure
Useful when exposures are rare
Useful in calculating risk and RR’s
Less expensive than interventional trial
Good when ethical issues limit use of intervention
Good for long induction/latent periods (retrospective)
Able to represent “temporality” (prospective)

Question 23

Advantages of Prospective Cohort Studies

Answer 23

Can obtain a greater amount of study-important info from patients
Follow-up/Tracking of patients may be easier
Better at giving answer to “temporality”
May look at multiple outcomes form a (supposed) single exposure
Can calculate incidence & incidence rates

Question 24

Disadvantages of Prospective Cohort Studies

Answer 24

Time, expense, and loss-to-follow-up’s
Not efficient for rare diseases
Not suited for long induction/latency conditions
Exposure (or its “amount”) may change over time

Question 25

Loss to Follow-Up

Answer 25

Possible with prospective cohorts
Lower sample size (power)
- increased risk of type 2 error
- loss of study participation may not be equal between groups
Do ALL you can (and think of) to limit LTFU’s

Question 26

Advantages of Retrospective Cohort Studies

Answer 26

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

Question 27

Disadvantages of Retrospective Studies

Answer 27

Requires access to charts, databases, employment records
Information may not factor in or control for other exposures to harmful elements
Patients may not be available for interview if contact necessary for missing/incomplete data
Exposure (or its “amount”) may have changed over time

Question 28

Issues affecting outcome occurrence in groups:

Answer 28

Level of exposure
Induction Period
Latency Period

Question 29

Induction period

Answer 29

Interval between exposure which causes disease and onset of disease-process

Question 30

Latency period

Answer 30

Interval between disease-process onset & clinical diagnosis of disease

Question 31

Key biases with Cohort Studies

Answer 31

Healthy-worker effect

Selection bias

Question 32

Matching

Answer 32

A way to strive to make groups as equal as possible on known/potential confounders
(Can match on a 1:1 or even higher (1:5) ratio [exposed to unexposed])