[2] Class 22-23: Cohort Studies

Question 1

Increasing evidence based on type of study:

Answer 1

Case-reports/series
Ecological 
Cross-sectional
Case-control
Cohort [strongest]

Question 2

Observational studies allowing researcher to be a passive observer of natural events occurring in naturally-exposed and unexposed groups.

Answer 2

Cohort studies

Question 3

Cohort group allocation can be based on 2 things:

Answer 3

Exposure-status

Or

Group membership [something in common]

Question 4

When is a cohort study useful?

Answer 4

When studying a rare exposure

Question 5

Other names for cohort studies:

Answer 5

Incidence, follow-up, longitudinal studies

Question 6

Cohort studies commonly generate a:

Answer 6

Risk of disease/outcome

Risk ratio

Question 7

On the 2x2 table, what do we know for cohort studies?

Answer 7

The column for totals. We know a+b and c+d

Question 8

4 reasons to use cohort study:

Answer 8

1. unable to force allocation [unethical]
2. limited resources [t,$,ppl]
3. exposure of interest is rare and little known about it and asso./outcomes
4. more interested in incident rates or risks for outcome of interest [than effects of interventions]

Question 9

In what way can cohort study be conducted?

Answer 9

Prospective, retrospective, and ambidirectional

Question 10

Cohort group design is based on:

Answer 10

Exposure

Question 11

Why is cohort considered higher evidence for observational studies than case-control?

Answer 11

Case-control is conducted retrospectively. cohort studies can be conducted prospectively

Question 12

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

Answer 12

Prospective cohort study

Question 13

At the start of the study, both exposure and outcome of interest have already occurred

‘In past, start at time of exposure and follow forward to outcome occurrence in present’

Answer 13

Retrospective cohort study

Question 14

In retrospective cohort studies, does the exposure have to come before outcome?

Answer 14

Yes in retrospective cohort studies, exposure still has to occur before the outcome.

Question 15

Ex: vietnam vets exposed to agent orange…looking at their outcomes in 2016

What type of study?

Answer 15

Retrospective cohort study

Question 16

Group allocation in cohort studies is based on:

What about in case-control studies?

Answer 16

exposure status;

Case-control group assignments based on disease-status

Question 17

Uses retrospective design to assess past differences but adds all data collected on additional outcomes prospectively forms start of study

Looking for outcomes in the past and into the future

Answer 17

Ambidirectional cohort study

Question 18

Group w/ something in common

Answer 18

Cohort

Question 19

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

Ex: Everyone born in KC limits in 2014

Answer 19

Birth cohort

Question 20

Individuals assembled at a given point based on some common factor

Ex: where ppl live or where they work

Answer 20

Inception cohort

Question 21

What is a inception cohort useful for?

Answer 21

Single-group assessments for incidence rate determination

Question 22

Individuals assembled based on some common exposure

Ex: frequency connected to environmental or other one-time event

Answer 22

Exposure cohort

Question 23

A cohort which cant gain [derived from irrevocable event]members but can have a loss to f/u

Answer 23

Fixed cohort

Question 24

A cohort that cant gain members or loss to f/u

Answer 24

Closed cohort

Question 25

A cohort w/ new additions and some loss to f/u

Can increase/decrease over time

Answer 25

Open/dynamic cohort

Question 26

What is the easiest part of cohort study design

Answer 26

Selecting the EXPOSED population

Question 27

What is the hardest part of cohort study design

Answer 27

Selection of the UNEXPOSED population

Question 28

How to select the exposed population in cohort studies:

It is based on

Answer 28

Allocates subjects based on PRE-DEFINED CRITERIA of “exposure”

Scientifically and consistently determined

Question 29

How to select unexposed population in cohort studies:

3 types:

Answer 29

Make groups as close as possible

Internal-best
General population
Comparison cohort

Question 30

Patients from 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 (control)

Ex: ford plant and exposure to paint

Answer 30

Internal cohort group unexposed selection—same cohort

Question 31

Used as a 2nd choice when the best-possible comparison group (internal) is not realistically possible

Answer 31

General population cohort selection for unexposed

Question 32

Least acceptable group

Simply attempt to match groups as close as possible on numerous personal characteristics (cant control for potentially harmful exposures in comparison cohort; also causing Dz)

Answer 32

Comparison cohort unexposed selection

Question 33

Strengths for cohort studies - in general

Answer 33

Assessing multiple outcomes of one exposure;
Useful when exposures are rare;
Useful in calc Risk and RR’s;
Less expensive than interventional trials;
Good when ethical issues limit use of interventional and long induction/latent periods;
Able to represent “temporality” (prospective)

Question 34

Weaknesses of cohort studies:

Answer 34

• not good for one outcomes
• not useful for common exposures
• not causal.
• not good for ethical use

Question 35

Advantages of PROSPECTIVE cohort studies:

Answer 35

• Can obtain more study-important info. From patients…more control over specific data collection process.
• f/u and tracking of pt.’s may be easier if you plan ahead
• better at giving answer to temporality
• may look at multiple outcomes from a single exposure
• can calc incidence/incidence rates

Question 36

Disadvantages of PROSPECTIVE cohort studies:

Answer 36

• Time, expense and LTFU
• not efficient for rare Dz
• Not suited for long induction/ latency conditions
• exposure may change over time

Question 37

Which cohort group is at risk to LTFU’s?

Answer 37

Prospective cohort studies

Question 38

What are the effects of LTFU’s on prospective cohorts:

Answer 38

Lowers sample size [power]

Increased risk of type II error

Loss of study participation/ f.u may not be = btw groups

Question 39

Must authors note in study the LTFU’s?

Answer 39

Authors must list LTFU’s by group (exposed/unexposed)

Question 40

What should be done to prevent LTFU’s?

Answer 40

EVERYTHING you can

Lots of time, energy, and resources

Question 41

Advantages of RETROSPECTIVE cohort studies:

Answer 41

• 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 42

Level of evidence of retrospective cohort

Answer 42

Case-control = retrospective cohort

Question 43

Disadvantages of retrospective cohort studies:

Answer 43

• Req’s access to charts, databases, employment records
• “info.” May not factor in or control for other exposures to harmful elements
• pt.’s may not be available for interview if contact necessary for missing or incomplete data
• exposure (or amt. of) may have changed over time

Question 44

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

Answer 44

Matching

This is different than just increasing numbers in the groups to make more comparable

Question 45

If healthy, you generally go to work (even if exposed). If too ill, you may be unemployed (maybe d/t exposure)

Answer 45

Healthy-worker effect

Question 46

How exposure status is defined/determined
(less of an issue w/ exposure status)

The groups made may not be representative of the population

Answer 46

Selection bias