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Flashcards in Cohort Study Deck (17)
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What is a Cohort Study?

Observational study allowing researchers to be passive observer of natural events occurring in naturally-exposed and unexposed groups


Cohort study designs include ____, ___ and ___.

Prospective, retrospective and ambidirectional


Why choose a cohort study?

Unable to force group allocation (unethical)
Limited Resources (time, money, subjects)
Rare exposure
Interested in incidence rates or risk for outcome of interest


Fixed cohort is when
Closed cohort is when
Open/Dynamic cohort is when

No members can be added but losses can occur
No additions, no losses
Open, losses and additions can occur anytime


Give some examples of Cohort.

Birth Cohort
Inception Cohort (COB)
Exposure Cohort (9/11)


Cohort studies can be designed to be divide groups based on

exposure status
Or group membership for "commonality"


Prospective study

Exposure group is selected on the basis of a past or current exposure and both groups followed into the future to assess for outcome of interest. No outcome known.

This study CAN show causation if precisely controlled with a well done design.


Retrospective Study

At the start of the study, both exposure and outcome of interest have already occurred but groups still allocated based on past history of exposure. Outcome known.

Lack of control since it already occurred, therefore no way to explain causation.



Using retrospective design to assess past differences but also add future data collected on additional outcomes PROSPECTIVELY.

Ex: Conducted study on exposure of Vietnam War vets on H1. Use medical records of past but still continue to follow them present day and future.


Selecting the exposure group is easier for Cohort studies since you are selecting based on ____

Pre-defined criteria


Selecting the unexposed group is harder for Cohort Studies but can be allocated from 3 sources:

General Pop.
Comparison Cohort(worst)


Strengths of Cohort Studies

Good for accessing multiple outcomes for one exposure
Good for Rare Exposures
Useful in calculating risk and risk ratios
Less expensive than interventional
When ethical issues limit use of interventional
Good for long induction/latent periods (retrospective)
Able to represent temporality


Weakness of Cohort Studies
(general but mostly retrospective)

Can't demonstrate causation (unless well controlled and design prospective type)

In retrospective type: hard to control for other exposures if more than one plausible for being associated with outcome

In retrospective type: can't control for other exposures or potential changes in amount of study exposure

Retrospective: An be impacted by unassessed confounders and biases

Retrospective: Limited Data

Prospective: Not good for long induction/latent periods


Strengths of Prospective Cohort Studies

Can obtain better data since we have control over specific data collection

Following up and tracking patients are easier

Good for temporarility

Look at multiple outcomes from a single exposure

Calculate incidence and incidence rate


Disadvantages of Prospective

Time, expense and lost-to-follow-ups
Not efficient for rare diseases
Not suited for long induction/latency conditions
Exposure may change over time


Strengths of Retrospective Cohort

Best for long induction/latency
Able to study rare exposures
Useful if data exists
Saves time and money to not conduct prospective


Weaknesses of Retrospective Cohort

Requires access to data which may/may not be thorough

Data may not control for other exposures

Exposure can change over time

Patient could be unavailable for interviewing if needed for filling in gaps for data.