cohort studies Flashcards
cohort studies
- observational studies allowing researcher to be a passive observer of natural event occurring in naturally-exposed and unexposed (comparison) groups
- group allocation based on exposure-status ….or….group membership (something in common), not group allocation-based
- useful when studying a rare exposure
- also termed: incidence studies/longitudinal studies
- retrospective, prospective, and ambidirectional
- commonly generates the risk of disease/outcome for each; risk ratio/relative risk is the measure of association
- already know exposure row totals (A+B and C+D) at start of study
cohort study groups flowchart
population
- exposure Y . -exposure N
1. disease/outcome Y . 1. disease/outcome Y
2. disease/outcome N . 2. disease/outcome N
why select a cohort study
- unable to force group allocation (randomize) due to ethical concerns or just not feasible
- limited resources
- exposure of interest is rare and little is known about its associations/outcomes
- if researcher is more interested in incidence rates or risk for outcome of interest (vs effects of interventions)
prospective cohort study
-exposure group is selected on the basis of a past or current exposure and both groups (exposed and non-exposed) are followed into the future to assess for outcome(s) of interest (which has yet to occur), and then compared
retrospective cohort study
- aka historical
- at start of study, both exposure and outcome of interest have already occurred, but groups still allocated based on past history of exposure
- start at time of exposure (historically) and follow forward to the point of outcome occurrence (known) in the present
- group allocation is based on exposure state, not disease status
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
definition of cohort
a group with something in common
birth cohort
individuals assembled based on being born in a geographic region in a given time period
e.g. everyone born in KC city limits in 2014
inception cohort
individuals assembled at a given point based on some common factor
e. g. where people live or work
- useful for single-group assessments for incidence rate determination
e. g. Nurses Health Study; Framingham Heart Study
exposure cohort
individuals assembled based on some common exposure
frequency connected to environmental or other one-time events
cohort sizes:
fixed: can’t gain members but can have loss-to-follow-ups; fixed at beginning
closed: closed on both ends (beginning and end); fixed cohort with no loss-to-follow-ups
open (dynamic): new additions and some loss-to-follow-ups; change as people immigrate and emigrate in and out of the population being studied; come and go
how to select exposed study population in cohort study
allocate subjects based on pre-defined criteria of ‘exposure”
how to select unexposed study population in cohort study
-make groups as close as possible (coming from the same cohort-not yet exposed)
unexposed groups can come from 3 sources
- internal (best): patients from same ‘cohort’ yet who are unexposed …..if not here (can’t find anybody who wasn’t exposed)…move to ……
- general population: (can become ‘internal group’ if exposed group came from all over);
- comparison cohort: simply attempt to match groups as close as possible on numerous personal characteristics (can’t control for other potentially harmful exposures in comparison cohort; also causing disease)
strengths of cohort studies
- good for assessing multiple outcomes of one exposure
- useful when exposure are rare (helpful in prospective)
- useful in calculating Risk and RR’s
- less expensive than interventional trials
- good when ethical issues limit use of interventional
- good for long induction/latent periods (retrospective)
- able to represent ‘temporality’ (prospective)
weaknesses of cohort studies
- can’t demonstrate causation (only well-controlled prospectives can approximate causation)
- hard to control for other exposures if more than one plausible for being associated with an outcome (retrospective)
- not good for long induction/latent period (prospective)
- can be impacted by unassessed confounders
- can be impacted by biases (selection and recall) (retrospective)
- limited by available data (retrospective)
advantages of prospective cohort studies
- can obtain a greater amount of study-important information from patients (more control over specific data collection process)
- follow-up/tracking of patients may be easier
- better at answering ‘temporality’
- may look at multiple outcomes from a (supposed) single exposure
- can calculate incidence and incidence rates
disadvantages of prospective cohort studies
- time, expense, and lost-to-follow-ups
- not efficient for rare diseases (use case-control for this)
- not suited for long induction/latency conditions
- exposure may change over time
loss-to-follow-up
- lowers sample size (Power)
- increased risk of type 2 error
- loss of study participation may not be equal between groups
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
disadvantages of retrospective cohort
- requires access to charts, databases, employment records (must be complete and thorough)
- 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 (or its amount) may have changed over time
baises in cohort studies
healthy-worker effect
selection bias: how exposure status is defined/determined
