[2] Class 22-23: Cohort Studies Flashcards
Increasing evidence based on type of study:
Case-reports/series Ecological Cross-sectional Case-control Cohort [strongest]
Observational studies allowing researcher to be a passive observer of natural events occurring in naturally-exposed and unexposed groups.
Cohort studies
Cohort group allocation can be based on 2 things:
Exposure-status
Or
Group membership [something in common]
When is a cohort study useful?
When studying a rare exposure
Other names for cohort studies:
Incidence, follow-up, longitudinal studies
Cohort studies commonly generate a:
Risk of disease/outcome
Risk ratio
On the 2x2 table, what do we know for cohort studies?
The column for totals. We know a+b and c+d
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]
In what way can cohort study be conducted?
Prospective, retrospective, and ambidirectional
Cohort group design is based on:
Exposure
Why is cohort considered higher evidence for observational studies than case-control?
Case-control is conducted retrospectively. cohort studies can be conducted prospectively
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
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
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.
Ex: vietnam vets exposed to agent orange…looking at their outcomes in 2016
What type of study?
Retrospective cohort study
Group allocation in cohort studies is based on:
What about in case-control studies?
exposure status;
Case-control group assignments based on disease-status
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
Group w/ something in common
Cohort
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
