Cohort Studies Flashcards
What are cohort studies?
- Observational studies allowing the researcher to passively observe natural events occurring in naturally exposed and unexposed (comparison) groups.
What are the two versions of a cohort study?
- Group-allocation based on exposure status
- Group membership because of something in common
What are some other names for cohort studies?
- Incidence studies
- Follow-up studies
- Longitudinal studies
What is commonly generated from cohort studies?
Risk of disease/outcome.
Risk Ratio/Relative Risk (RR) as a measure of association
What are some reasons for selecting a cohort study?
- Unable to force group allocation
- Limited resources
- The exposure of interest is rare in occurrence and little is known about its associations/outcomes (v1)
- More interested in incidence rates or risks for outcome interest (more than effects of interventions)
What are the 3 fashions/perspectives that a cohort study can be conducted in?
- Prospective
- Retrospective
- Ambidirectional
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 and then compared
Retrospective (Historical) Cohort Studies
- At the start of the study both exposure and outcome of interest have already occurred, but groups allocated based on past history of exposure.
- Retrospectively start at time of exposure and follow forward to the point of outcome occurrence (known) in the present
- exposure still has to happen before outcome of interest and group allocation is based on exposure status
Ambidirectional Cohort Studies
- Uses retrospective design to assess past differences (up to present, but adds future data collected on additional outcomes prospectively from start of study.
- looking for outcomes in the past and into the future
What are the types of cohorts that refer to a group with something in common?
- Birth Cohort
- Inception Cohort
- Exposure Cohort
Birth Cohort
- Individuals assembled based on being born in a geographic region in a given period of time
Inception Cohort
- Individuals assembled at a given point based on some common factor
- Where people live, work, or something they have in common
- Useful for single-group assessments for incidence rate determination
Exposure Cohort
- Individuals assembled based on some common exposure
- frequency connected to environmental or other one-time events
Fixed Cohort
A cohort which can’t gain members but can have loss-to-follow-ups
Closed Cohort
A fixed cohort with no loss-to-follow-ups
Open (or Dynamic) Cohort
- Cohort with new additions and some loss-to-follow-ups
- Can increase or decrease over time, as people emigrate or immigrate in and out of cohort (population) being studied
How do you select an unexposed study population?
Make the groups as close as possible
What 3 sources can the unexposed group come from?
- Internal
- General Population
- Comparison Cohort
Internal Source
Patients from the same cohort, who are unexposed
- best, if feasible
General Population Group
- used as a second choice when the internal is not realistically possible
(E.g. Everyone is exposed)
Comparison Cohort
- Least acceptable group
- simply attempt to match groups as close as possible on numerous personal characteristics
What are the strengths of cohort study designs?
- good for assessing multiple outcomes of one exposure
- Useful when exposure are rare
- Useful in calculating Risks and RR
- Less expensive than interventional
- Good when ethical issue limit use of interventional
- Good for long Induction/Latent periods (retro)
- Able to represent temporality (Pro)
What are some advantages of Prospective Cohort Studies?
- Can obtain a greater amount of study-important information from patients
- Follow up or tracking patients may be easier
- Better at giving answer to temporality
- May look at multiple outcomes from a (supposed) single exposure
- Can calculate incidence & incidence rates
Disadvantages of Prospective Cohort Studies
- Time, Expense, & Lost-to-follow-up
- Not efficient for rare diseases
- Not suited for long induction/latency conditions
- Exposure may change over time
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
Disadvantages of Retrospective
- Requires access to charts, databases, employment records
- information may not factor in or control for 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 be changed over time
Matching
A way to make groups as equal as possible on known/potential confounders
Key biases with cohort studies
- Healthy worker effect
- Selection bias
