Classes 22-23 Cohort Studies Flashcards
Cohort studies
Observational, analytical studies allowing researcher to be a passive observer of natural events occurring in naturally-exposed and unexposed (comparison) groups
In Cohort studies, group-allocation is based on:
Exposure-status or group membership (something in common)
When are Cohort studies useful?
Useful when studying a rare exposure
What are other names for Cohort studies?
Incidence studies
Follow-up studies
Longitudinal studies
Cohort studies commonly generate the ____ ____ as a measure of association
Risk Ratio (RR)
Reasons to select a Cohort design
Unable to “randomize”
Limited resources
Exposure of interest is rare
More interested in incidence rates/predictors of or risks for outcome of interest
True or False:
Cohort studies can only be done in a prospective fashion.
False
They can be conducted in a prospective, retrospective, or ambidirectional fasion.
Prospective Cohort Studies
Exposure group is selected on the basis of a past or current exposure and both groups followed into the future to assess for outcome(s) of interest, and then compared
Retrospective Cohort Studies
At the start of the study, both the exposure and the outcome of interest have occurred
Exposure still has to occur before outcome of interest & group allocation is based on exposure status, not disease status
Ambidirectional Cohort Studies
Uses retrospective design to assess past differences but adds all data collected on additional outcomes prospectively from start of study (looking for outcomes in the past and into the future)
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 (ex. Framingham study and Nurses’ health study)
Exposure Cohort
Individuals assembled based on some common exposure (frequently connected to environmental or other one-time event)
Fixed Cohort
A cohort (derived from an irrevocable event) 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
A cohort with new additions and some loss-to-follow-ups (can increase or decrease over time)
How to select an Exposed study population
The easier part
Allocate subject based on pre-defined criteria of “exposure”
How to select an Unexposed study population
Make the groups as close as possible (coming from the same cohort/population (yet not exposed))
Can come from 3 sources: Internal, general population, or comparison cohort
Unexposed group from INTERNAL source
Patients from the same “cohort”, yet who are unexposed (most similar)
If there are only levels of exposure, you may have to use the lowest exposure group as comparator
Unexposed group from GENERAL POPULATION source
Used as a second choice when the best-possible comparison group (internal) is not realistically possible (e.g., everyone is exposed; or the exposure subjects were drawn from the general population)
Randomly selecting
Unexposed group from COMPARISON COHORT source
Least acceptable group
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 exposures are rare
Useful in calculating risk and RR’s
Less expensive than interventional trial
Good when ethical issues limit use of intervention
Good for long induction/latent periods (retrospective)
Able to represent “temporality” (prospective)
Advantages of Prospective Cohort Studies
Can obtain a greater amount of study-important info from patients
Follow-up/Tracking of patients may be easier
Better at giving answer to “temporality”
May look at multiple outcomes form a (supposed) single exposure
Can calculate incidence & incidence rates
Disadvantages of Prospective Cohort Studies
Time, expense, and loss-to-follow-up’s
Not efficient for rare diseases
Not suited for long induction/latency conditions
Exposure (or its “amount”) may change over time
Loss to Follow-Up
Possible with prospective cohorts
Lower sample size (power)
- increased risk of type 2 error
- loss of study participation may not be equal between groups
Do ALL you can (and think of) to limit LTFU’s
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 Studies
Requires access to charts, databases, employment records
Information may not factor in or control for other exposures to harmful elements
Patients may not be available for interview if contact necessary for missing/incomplete data
Exposure (or its “amount”) may have changed over time
Issues affecting outcome occurrence in groups:
Level of exposure
Induction Period
Latency Period
Induction period
Interval between exposure which causes disease and onset of disease-process
Latency period
Interval between disease-process onset & clinical diagnosis of disease
Key biases with Cohort Studies
Healthy-worker effect
Selection bias
Matching
A way to strive to make groups as equal as possible on known/potential confounders
(Can match on a 1:1 or even higher (1:5) ratio [exposed to unexposed])
