Cohort studies (Lecture 4)

Question 1

Define a cohort study

Answer 1

observational studies that allow a researcher to be a passive observer of natural events occurring in a naturally exposed and unexposed groups

Question 2

What is the group allocation of the cohort studies based on?

Answer 2

1. exposure status
   OR
2. group membership

Question 3

When are cohort studies useful?

Answer 3

When studying a rare exposure

Question 4

What are two other names for a cohort study?

Answer 4

Incidence studies and longitudinal studies

Question 5

What two measures of association are typically used in a cohort study?

Answer 5

risk of the disease or the outcome and the risk ratio or the relative risk

Question 6

What are the reasons to pick a cohort study?

Answer 6

• unable to randomize or force the group allocation
• limited resources
• exposure of interest is rare
• more interested in the incidence or risk rates

Question 7

What are the 3 ways in which a cohort study can be conducted in?

Answer 7

prospective
retrospective
ambidirectional

Question 8

Define a prospective cohort study

Answer 8

exposure group is selected on the bases of past or current exposure and both groups are following into the future to assess for the outcomes

Question 9

Define a Retrospective cohort study

Answer 9

At the start of the study, the exposure and the source has already occurred bit the groups are still allocated based on the past history of the exposure

Question 10

What is another word for a retrospective study?

Answer 10

historical

Question 11

Where do you start at in a retrospective study?

Answer 11

At the time of exposure (historically- it is in the past) and then you follow it forward to the point of outcome that is in the present

*exposure has to occur before the outcome of interest and the group allocation is based on exposure status, not the disease status

Question 12

Define an ambidirectional cohort study

Answer 12

Uses retrospective design to assess past differences but also adds future data collected on additional outcomes prospectively from the start of the study

Question 13

How is an ambidirectional study considered to be bidirectional?

Answer 13

Looking for outcomes that are in the past and that are known in the present but also looks into the future

Question 14

Define cohort

Answer 14

reders to a group with something in common

Question 15

Define birth cohort

Answer 15

individuals that are grouped together based on being born in a geographic region in a given time period

Question 16

Define inception cohort

Answer 16

Individuals that are grouped together at a given point based on a common factor
-where people live, where they work, something they have in common

Question 17

Define exposure cohort

Answer 17

Individuals are assembled together based on a common exposure

Question 18

True or false

Cohort changes cannot change over time

Answer 18

False: they may or may not change over time

Question 19

Define a fixed cohort

Answer 19

a cohort which cannot gain members but can have loss to follow ups

Question 20

Define a closed cohort

Answer 20

A fixed cohort with no loss to f/u

Question 21

Define an open (dynamic) cohort

Answer 21

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

Question 22

How do you select the exposed study populations?

Answer 22

You allocate the subjects based on a predefined criteria of exposure

Question 23

What are the goals of an unexposed study population?

Answer 23

To make the groups as close as possible (coming from the same cohort while also being unexposed)

Question 24

What are the 3 sources that an unexposed group can come from?

Answer 24

1. Internal
2. General population
3. Comparison cohort

Question 25

Describe the internal source of unexposed selection

Answer 25

patients come from the same “cohort” but are unexposed
If there are levels of exposure, use the lease exposed group as a comparator

BEST

Question 26

Describe the general population selection group for the unexposed study population

Answer 26

used if the internal source is not available

everyone is exposed; the exposure subjects came from the general population

Question 27

Describe the comparison cohort source of an unexposed selection

Answer 27

This is the LEAST acceptable
attempts to match the groups as close as possible on personal characteristics

cannot control for the other potentially harmful exposures in the comparison cohort

Question 28

What are the strengths of a cohort study?

Answer 28

1. Multiple outcomes
2. rare exposures
3. calculating risk and RR
4. ethical issues are limiting the interventional, can use cohort
5. long conduction/latent periods (retro)
6. “temporality” (prospective)

Question 29

What are the weaknesses of a cohort study?

Answer 29

1. Cannot demonstrate causation
2. hard to control for other exposures if more than one plausible for being associated with an outcome
3. retrospective cannot control for other exposures or potential changes in amount of study exposure
4. not good for long induction /latent period (prospective; retrospective is better)
5. impacted by unassisted confounders
6. impacted by biases (selection and recall)
7. limited by available data (retrospective; less for prospective)

Question 30

What are the advantages of a Prospective cohort study?

Answer 30

1. can obtain a greater greater amount of info from patients
   - more control over specific data collection
2. Follow up may be easier
3. Temporality
4. Multiple outcomes for a single exposure
5. Can calculate incidence and incidence rates

Inclusive- info 
Friends- Follow up 
Talk to- temporality 
Many-multiple
Individuals-Incidence

Question 31

What are the disadvantages of a prospective cohort study?

Answer 31

1. Time, expense, loss to fu
2. not efficient for rare
3. not good for long induction
4. exposure may change over time

Tyranosaurus (T)- Time
Rexes (rare diseases)
Love (long induction no bueno)
Eating (exposure changes)

Question 32

What is the main concern of loss to follow up in prospective studies

Answer 32

The loss of participation MAY NOT be equal in all the groups

Question 33

What are the advantages of the retrospective studies?

Answer 33

1. good for long induction
2. rare exposures can be studied
3. useful for existent data
4. saves time and money compared to prospective

Little (long)
Racoons (rare)
Deviously (data-existent)
Steal (saves)

Question 34

What are the disadvantages of the retrospective studies?

Answer 34

1. access to charts and records
2. “information” may not factor in or control for other exposures
3. Patients may not be available to contact if something is missing
4. Exposure may change

Are (access)
Iguanas (information)
People (patients no contact)
Eaters? (exposure)

Question 35

Define matching

Answer 35

a way to make the groups as equal as possible BASED on the known confounders

Question 36

Define the healthy worker effect

Answer 36

IF you are studying a work environment, the people that you get in the office may be healthy because they haven’t worked there for very long or have not been exposed to whatever is of concern enough. You are missing the people who are the experienced workers who may be too sick to come to work or dead

Question 37

Define selection bias

Answer 37

How the exposure status is defined could result in some selection bias (exposure vessels level of exposure etc)