[2] Class 22-23: Cohort Studies Flashcards by John Pate

Increasing evidence based on type of study:

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

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

Cohort studies

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Cohort group allocation can be based on 2 things:

Exposure-status

Group membership [something in common]

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When is a cohort study useful?

When studying a rare exposure

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Other names for cohort studies:

Incidence, follow-up, longitudinal studies

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Cohort studies commonly generate a:

Risk of disease/outcome

Risk ratio

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

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

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4 reasons to use cohort study:

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

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In what way can cohort study be conducted?

Prospective, retrospective, and ambidirectional

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Cohort group design is based on:

Exposure

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

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

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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:

Prospective cohort study

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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’

Retrospective cohort study

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

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

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

What type of study?

Retrospective cohort study

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Group allocation in cohort studies is based on:

What about in case-control studies?

exposure status;

Case-control group assignments based on disease-status

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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

Ambidirectional cohort study

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Group w/ something in common

Cohort

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

Ex: Everyone born in KC limits in 2014

Birth cohort

Individuals assembled at a given point based on some common factor

Ex: where ppl live or where they work

Inception cohort

What is a inception cohort useful for?

Single-group assessments for incidence rate determination

Individuals assembled based on some common exposure

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

Exposure cohort

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

Fixed cohort

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

Closed cohort

A cohort w/ new additions and some loss to f/u Can increase/decrease over time

Open/dynamic cohort

What is the easiest part of cohort study design

Selecting the EXPOSED population

What is the hardest part of cohort study design

Selection of the UNEXPOSED population

How to select the exposed population in cohort studies: It is based on

Allocates subjects based on PRE-DEFINED CRITERIA of "exposure" Scientifically and consistently determined

How to select unexposed population in cohort studies: 3 types:

Make groups as close as possible Internal-best General population Comparison cohort

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

Internal cohort group unexposed selection---same cohort

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

General population cohort selection for unexposed

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)

Comparison cohort unexposed selection

Strengths for cohort studies - in general

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)

Weaknesses of cohort studies:

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

Advantages of PROSPECTIVE cohort studies:

- 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

Disadvantages of PROSPECTIVE cohort studies:

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

Which cohort group is at risk to LTFU's?

Prospective cohort studies

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

Lowers sample size [power] Increased risk of type II error Loss of study participation/ f.u may not be = btw groups

Must authors note in study the LTFU's?

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

What should be done to prevent LTFU's?

EVERYTHING you can Lots of time, energy, and resources

Advantages of RETROSPECTIVE cohort studies:

- 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

Level of evidence of retrospective cohort

Case-control = retrospective cohort

Disadvantages of retrospective cohort studies:

- 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

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

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

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

Healthy-worker effect

How exposure status is defined/determined (less of an issue w/ exposure status) The groups made may not be representative of the population

Selection bias