Cohort Studies Flashcards
What are cohort studies?
observational studies allowing researcher to be a passive observer of natural events occurring in naturally exposed and unexposed (comparison) groups
In cohort studies, what is group allocation based on?
exposure status or group membership (something in common)
When are cohort studies useful?
when studying a rare exposure
What are the reasons to select a cohort design?
unable to force group allocation
limited resources
exposure of interest is rare
more interested in incidence rates or risks for outcome of interest
What ways can cohort studies be conducted?
prospective, retrospective, ambidirectional
What is a prospective cohort study?
exposure group is selected on the basis of a past or current exposure and both groups followed into future to assess for outcomes of interest, then compared
What is a retrospective cohort study?
at the state of the study both exposure and outcome of interest of already occurred, but groups still allocated based on past history of exposure
What is an ambidirectional cohort study?
uses retrospective design to assess past differences (up to present), but also adds future data collected on additional outcomes prospectively from start of study
What is a cohort?
group with something in common
What is a birth cohort?
individuals assembled based on being born in a geographic region in a give time period
What is an inception cohort?
individuals assembled at give point based on some common factor
What is an exposure cohort?
individuals assembled based on some common exposure
What is a fixed cohort?
cohort which can’t gain members but can have loss to follow ups
What is a closed cohort?
a fixed cohort with no loss to follow ups
What is an open or dynamic cohort?
a cohort with new additions and some loss to follow ups
How do you select an exposed study population in cohort studies?
allocate subjects based on pre defined criteria of exposure
How do you select unexposed study population in cohort studies?
make groups as close as possible (coming from same cohort/population, yet not exposed)
What are the 3 sources an unexposed group can come from?
internal
general population
comparison cohort
What is an internal source?
patients from the same cohort, yet are unexposed
What is the general population source?
used as second choice, unexposed subjects drawn from general population
What is a comparison cohort source?
least acceptable group
simply attempt to match groups as close as possible on numbers personal characteristics (can’t control for other potentially harmful exposures in comparison cohort, also causing disease)
What are the strengths to cohort studies?
- good at assessing multiple outcomes of one exposure
- useful when exposures are rare
- useful in calculating risk and risk ratios
- less expensive
- good when ethical issues limit interventional
- good for long induction/latent periods (retrospective)
- able to represent temporality
What are the weaknesses of cohort studies?
- can’t demonstrate causation
- hard to control for other exposures if more than one plausible for being associated with an outcome
- retrospective can’t control for other exposures or potential changes
- not good for long induction/ latent periods (prospective)
- can be impacted by unassessed confounders
- can be impacted by various biases
- limited by available data
What are the advantages to prospective cohort studies?
- can obtain a greater amount of study important information from patients
- follow up/ tracking of patients may be easier
- better at giving answer to temporality
- may look at multiple outcomes form a single exposure
- can calculate incidence and incidence rate
What are the disadvantages to prospective cohort studies?
- time, expense, and lost to follow up
- not efficient for rare diseases
- not suited for long induction/ latency conditions
- exposure (or its amount) may change over time
When it comes to loss to follow up, do all you can to ____
limit them
What are the advantages to 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
What are the disadvantages to retrospective cohort studies?
- require access to charts, databases, employment records, etc.
- information may not factor in or control for other exposures to harmful elements during study period or over time
- patients may not be available for interview if contact necessary for missing or incomplete data
- exposure may have changed over time
What are the key biases in cohort studies?
health worker effect
selection bias
