Cohort studies Flashcards
Cohort studies definition
Observational studies allowing researcher to be a passive observer of natural events 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
Other names for cohort studies
Incidence studies/longitudinal studies
Reasons to select cohort studies
Unable to force group allocation
Limited resources
Exposure of interest is rare
More interested in incidence rates or risks for outcome of interest
Cohort studies can be conducted in which fashions
Prospective, retrospective, or ambidirectional fashion
Prospective cohort studies
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 (which has yet to occur)
Retrospective cohort studies
At the start of the study, both exposure and outcome of interest have already occurred, but groups still allocated based on past history of exposure
Exposure still must occur before outcome
Ambidirectional cohort studies
Uses retrospective design to assess past difference, but also adds future data collected on additional outcomes prospectively from start of study
–Looking for outcomes in the past, and known in the present, but also in the future
Cohort also refers to
A group with something in common
Birth cohort
Individuals assembled based on being born in a geographic region in a given time period
Inception cohort
Individuals assembled at a given point based on some common factor
- where they live/work
- useful for single group assessments for incidence rate determination
Exposure cohort
Individuals assembled based on some common exposure
Fixed cohort
A cohort which cant gain members but can have loss to follow ups
Closed cohort
A fixed cohort with no loss to follow ups
Open/dynamic cohort
A cohort with new additions and some lost to follow ups
Unexposed group can come from what three sources
Internal (best source) General population (2nd choice) Comparison cohort (least accepted)
Strengths of retrospective cohort studies
Best for long induction/latency periods
Able to study rare conditions
Useful if the data already exists
Saves time and money compared to prospective
Weaknesses of retrospective cohort studies
Cant demonstrate causation
Hard to control for other exposures
Must have access to old records
Can be impacted by unassessed confounders and various biases (selection/recall/assessment bias)
May not be able to interview study subjects
Weakness of prospective cohort
Not good for long induction/latent periods
Time, expense, lost to follow up
Not efficient for rare diseases
Exposure may change over time
Strengths of prospective cohort studies
Can obtain larger amount of study-important info from patients
Follow-up/tracking of patients easier
Better at giving answer to temporality
May look at multiple outcomes from single exposure
Calculate incidence and incidence rates
Key biases with cohort studies
Healthy-worker bias
Selection bias
