Cohort studies (Lecture 4) Flashcards
Define a cohort study
observational studies that allow a researcher to be a passive observer of natural events occurring in a naturally exposed and unexposed groups
What is the group allocation of the cohort studies based on?
- exposure status
OR - group membership
When are cohort studies useful?
When studying a rare exposure
What are two other names for a cohort study?
Incidence studies and longitudinal studies
What two measures of association are typically used in a cohort study?
risk of the disease or the outcome and the risk ratio or the relative risk
What are the reasons to pick a cohort study?
- unable to randomize or force the group allocation
- limited resources
- exposure of interest is rare
- more interested in the incidence or risk rates
What are the 3 ways in which a cohort study can be conducted in?
prospective
retrospective
ambidirectional
Define a prospective cohort study
exposure group is selected on the bases of past or current exposure and both groups are following into the future to assess for the outcomes
Define a Retrospective cohort study
At the start of the study, the exposure and the source has already occurred bit the groups are still allocated based on the past history of the exposure
What is another word for a retrospective study?
historical
Where do you start at in a retrospective study?
At the time of exposure (historically- it is in the past) and then you follow it forward to the point of outcome that is in the present
*exposure has to occur before the outcome of interest and the group allocation is based on exposure status, not the disease status
Define an ambidirectional cohort study
Uses retrospective design to assess past differences but also adds future data collected on additional outcomes prospectively from the start of the study
How is an ambidirectional study considered to be bidirectional?
Looking for outcomes that are in the past and that are known in the present but also looks into the future
Define cohort
reders to a group with something in common
Define birth cohort
individuals that are grouped together based on being born in a geographic region in a given time period
Define inception cohort
Individuals that are grouped together at a given point based on a common factor
-where people live, where they work, something they have in common
Define exposure cohort
Individuals are assembled together based on a common exposure
True or false
Cohort changes cannot change over time
False: they may or may not change over time
Define a fixed cohort
a cohort which cannot gain members but can have loss to follow ups
Define a closed cohort
A fixed cohort with no loss to f/u
Define an open (dynamic) cohort
A cohort with new additions and some loss to f/u
How do you select the exposed study populations?
You allocate the subjects based on a predefined criteria of exposure
What are the goals of an unexposed study population?
To make the groups as close as possible (coming from the same cohort while also being unexposed)
What are the 3 sources that an unexposed group can come from?
- Internal
- General population
- Comparison cohort
Describe the internal source of unexposed selection
patients come from the same “cohort” but are unexposed
If there are levels of exposure, use the lease exposed group as a comparator
BEST
Describe the general population selection group for the unexposed study population
used if the internal source is not available
everyone is exposed; the exposure subjects came from the general population
Describe the comparison cohort source of an unexposed selection
This is the LEAST acceptable
attempts to match the groups as close as possible on personal characteristics
cannot control for the other potentially harmful exposures in the comparison cohort
What are the strengths of a cohort study?
- Multiple outcomes
- rare exposures
- calculating risk and RR
- ethical issues are limiting the interventional, can use cohort
- long conduction/latent periods (retro)
- “temporality” (prospective)
What are the weaknesses of a cohort study?
- Cannot demonstrate causation
- hard to control for other exposures if more than one plausible for being associated with an outcome
- retrospective cannot control for other exposures or potential changes in amount of study exposure
- not good for long induction /latent period (prospective; retrospective is better)
- impacted by unassisted confounders
- impacted by biases (selection and recall)
- limited by available data (retrospective; less for prospective)
What are the advantages of a Prospective cohort study?
- can obtain a greater greater amount of info from patients
- more control over specific data collection - Follow up may be easier
- Temporality
- Multiple outcomes for a single exposure
- Can calculate incidence and incidence rates
Inclusive- info Friends- Follow up Talk to- temporality Many-multiple Individuals-Incidence
What are the disadvantages of a prospective cohort study?
- Time, expense, loss to fu
- not efficient for rare
- not good for long induction
- exposure may change over time
Tyranosaurus (T)- Time
Rexes (rare diseases)
Love (long induction no bueno)
Eating (exposure changes)
What is the main concern of loss to follow up in prospective studies
The loss of participation MAY NOT be equal in all the groups
What are the advantages of the retrospective studies?
- good for long induction
- rare exposures can be studied
- useful for existent data
- saves time and money compared to prospective
Little (long)
Racoons (rare)
Deviously (data-existent)
Steal (saves)
What are the disadvantages of the retrospective studies?
- access to charts and records
- “information” may not factor in or control for other exposures
- Patients may not be available to contact if something is missing
- Exposure may change
Are (access)
Iguanas (information)
People (patients no contact)
Eaters? (exposure)
Define matching
a way to make the groups as equal as possible BASED on the known confounders
Define the healthy worker effect
IF you are studying a work environment, the people that you get in the office may be healthy because they haven’t worked there for very long or have not been exposed to whatever is of concern enough. You are missing the people who are the experienced workers who may be too sick to come to work or dead
Define selection bias
How the exposure status is defined could result in some selection bias (exposure vessels level of exposure etc)
