22-23 Cohort Studies Flashcards
How are cohort studies setup?
based on exposure, looking for outcome
Define cohort studies.
observational analytical studies allowing researchers to be passive observers of natural events occurring in naturally-exposed and unexposed (comparison) groups
What are cohort studies also termed?
incidence studies, follow-up studies, longitudinal studies
When are cohort studies useful?
when studying a rare exposure
What does cohort studies commonly generate?
risk ratios as a measure of association
Can cohort studies be conducted in a prospective, retrospective, or ambidirectional fashion?
yes
List the reasons not to chose a cohort study
Unable to randomize
limited resources
the exposure of interest is rare in occurrence and little is known about its associations/causes
more interested in incidence rates/predictors of or risks for outcome of interest
Describe a prospective cohort study.
exposure group is selected on the basis of a past or current exposure and both groups followed into the future to assess for outcomes of interest and then compared
Describe a retrospective cohort study.
at the start of the study, both the exposure and the outcome of interest have occurred
- retrospectively start at time of exposure and follow forward to the point of outcome occurrence in the present
- exposure still has to occur before outcome of interest and group allocation is based on exposure status, not disease status
Describe an ambidirectional cohort study.
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
What does a cohort study refer to?
a group with something in common
Give examples of cohort studies.
Birth cohort
Inception cohort
exposure cohort
Describe birth cohort.
individuals assembled based on being born in a geographic region in a given time period
Describe inception cohort.
individuals assembled at a given point based on some common factor
- where people live, work, etc.
When is inception cohort studies useful?
for single-group non-comparisons for incidence rate determination
Describe an exposure cohort study.
Individuals assembled based on some exposure
- frequency connected to environmental or other one-time events
Give the different types of cohort studies.
Fixed: can’t gain members but can have loss to follow ups
Closed: fixed with no loss to follow ups
open: new additions and some loss to follow ups; can increase or decrease over time
How would you select an exposed study population?
allocate subject based on pre-defined criteria of exposure
How would you select an unexposed study population?
make the groups as close as possible
unexposed group can come from 3 sources
- internal population, general population, comparison cohort
Describe internal population.
Patients are from the same “cohort” yet who are unexposed
if there are only levels of exposure you have to use the lowest exposure group as comparator
When would you use a general population?
when everyone is exposed, or the exposure subjects were drawn from the general population
What does a comparison cohort attempt to do?
match groups as close as possible on numerous personal characteristics
Can a comparison cohort study control for other potentially harmful exposures in comparison?
No
List the 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 trials
good when ethical issues limit use of interventional studies
good for long induction/latent periods (retrospective)
able to represent “temporality” (prospective)
What are the advantages of 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 from a single exposure
can calculate incidence & incidence rates
What are the disadvantages of prospective cohort studies?
time, expense, & lost to follow up’s
not efficient for rare diseases
not suited for long induction/latency conditions
exposure may change over time
What is the effect of loss to follow ups?
lowers sample size
increase risk of type 2 error
study population may not be equal between groups
What are the 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 cohort studies
What are the disadvantages of retrospective cohort 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 or incomplete data
exposure may have changed over time
What are the issues affecting outcome occurrence in groups?
level of exposure: stratify
Induction Period
Latency period
Define induction period.
interval between exposure which causes disease and onset of disease-process
Define latency period.
interval between disease-process onset & clinical diagnosis of disease
What is one way researcher strive to make the groups as equal as possible on known/potential confounders in cohort studies?
matching
List and describe the key biases with cohort studies.
Healthy-worker effect
- if healthy, you work (even if exposed). if too ill to work (due to exposure?)you may be unemployed (now part of non-working general population).
Selection bias
- how exposure status is defined/determined (less of an issue with exposure status)
Of all the study designs, which one is the only one that touch at the concept of temporality?
cohort studies
Give an example of a fixed cohort study.
first responders of 9/11
Give an example of a closed cohort study.
short term studies
Give an example of an open cohort study.
Framingham Heart Study
