Cohort Studies Flashcards
relate cohort studies to strength in evidence of all observational studies
has the most evidence
only observational that can be a controlled prospective study
cohort studies are _______ studies that allow the researcher to be …..
observational
a passive observer of natural events occurring in naturally exposed or unexposed groups
group allocation is based upon?
exposure status
or
group membership—simply something in common
cohort studies are useful when studying ?
a rare exposure
other terms for cohort studies
incidence studies
longitudinal studies
what measures of association do cohort studies calculate?
- risk of disease/outcome
2. RR/relative risk
define cohort
a group of people who simply have something in common
and then you can further allocate them into groups based upon exposure or not/ or disease/ not diseased
reasons to choose a cohort design
- unable to randomize
- limited resources
- exposure of interest is rare
- more interested in incidence rate or risk of outcome than an intervention
exposure of interest is rare
- -rarely occurs
- -little is known about its associations or outcomes
time line direction of cohort studies
can be prospective, retrospective, or ambidirectional fashion
prospective cohort studies
- exposure groups is selected based on past or present exposure
- both groups are followed into future to assess for outcomes
historical cohort study = ?
retrospective design
retrospective cohort studies
- at start of study exposure and outcome has already occurred
- but groups are still allocated based on past history of exposure
- not disease status
ambidirectional design
- -uses past exposure for group allocation
- -adds present data and
- –keeps going in a prospective fashion
birth cohort
–individuals assembled into a cohort based upon being born in same region/in same time period
inception cohort
–individuals assembled in a cohort at a specific time based upon a common factor
–ex. cob class– all in this grad program
exposure cohort
- -brought together based upon some common exposure
ex. 9/11 responders – exposed to carcinogens
cohort sizes may ______ over time
not change
cohort size control
- fixed cohort
- closed cohort
- open/dynamic cohort
fixed cohort
a cohort which cannot gain members but can have lost to follow-up
closed cohort
a fixed cohort but no loss to follow-up
open cohort
- -dynamic cohort
- –a cohort w/ new additions and some loss to follow-up
- ex. nurse health
moving, death, etc.
sources of the unexposed group
- internal
- general population
- comparison cohort
internal source
- best option
- patients from same cohort yet unexposed
- use lowest exposure group when levels of exposure exist
describe levels of exposure
–when exposure intensity increases/decreases over time
- -in some events there are no unexposed subjects
- -comparison group may just have to be the lowest exposure people
general population
- a second choice when an internal group is not available
comparison cohort
- the least favorable group
2. simply match groups as close as possible on personal characteristics
strengths of cohort design
- good for multiple outcomes of one exposure
- rare exposures
- calculate risk and RR
- less expensive
- less ethical limits
- good for retrospective
- can be prospective
matching in cohort studies
–a way to strive to make groups as equal as possible for known confounders
key biases in cohort studies
healthy worker effect
selection bias
weaknesses of cohort studies
- can’t show causality
- can’t control variables in retro
- retro can’t control for other exposures
- prospec not good for long induction periods
- impacted by unknown confounders
- effected by biases
- limited by available data when retro
advantages of prospective cohort studies
- obtain greater amount of specific data
- tracking subjects easier
- can assess temporality
- can eval multiple outcomes of one exposure
- calc incidence risk and rate
disadvantages of prospective cohort studies
- time, money, lost to follow up
- not great for rare diseases
- not suited for long induction or latent periods
- exposure can change – levels of exposure
lost to follow up in prospective designs
- -a problem just like in interventional designs
- -increases risk of type 2 error
advantages of retrospective cohort designs
- best for long induction/latent periods
- study rare exposures
- useful when data already exists
- saves time and money compared to prospec
disadvantages of retrospective cohort designs
- requires access to data
- cannot control for variables or other exposures
- patients might not be available for contact
- exposure can change over time – levels
