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What is a cohort? (2)
- any designated group of person who are followed or traced over a period of time
- a group of individuals with a common characteristic or experience
What do you compare in cohort studies?
compare occurrence of disease in "exposed" and "unexposed" people
Where do we get the exposure information from in a cohort study?
exposure information from BEFORE people develop the disease
How are participants collected in prospective cohort? (2)
- 1. select study groups on the basis of exposure
- 2. select a population before identifying who is exposed/unexposed
What do you compare in prospective cohort studies?
compare the rates of the outcome in the exposed and unexposed groups
How is data collected in a prospective cohort study? What is an example?
- prospectively, moving forward in time
- Example: via mailed questionnaires during study period
What are the steps in conducting a prospective cohort study? (4)
- select the study population
- identify the exposed and unexposed groups
- follow both groups forward in time and wait for the outcome to develop
- compare the rates of the outcome in both groups
How are the exposed and unexposed groups in retrospective cohort studies selected?
retrospectively, in the past
What data does retrospective cohort studies use?
Where does the data from retrospective studies come from? (4)
- occupational databases
- hospital administration data
- health insurance databases
- health registries
In retrospective cohort studies, how is disease occurrence usually measured?
cumulative incidence or incidence rate
How is the relationship between exposed and unexposed groups in retrospective cohort studies quantified?
with RR (risk ratio)
how can you prevent selection bias in retrospective cohort studies?
study groups should be similar in every respect other than the exposure
What is information bias in retrospective cohort?
when information collected in exposed group (ex.type of info) differs from the information collected from unexposed
A cohort study is conducted to assess the association between asthma and the development of ADHD. How can you solve misclassification of outcome? (2)
- mask the assessor
- objective outcomes
What is loss to follow up?
exposed individuals are more likely to be lost to follow up, compared with unexposed individuals
Why might participants be lost to follow up? (3)
- might move
- might lose interest
What are three things you can do to prevent loss to follow up?
- try to avoid it
- select a study population to those that are likely to remain enrolled for the entire follow up period
- maintain up to date contact information
In prospective cohort study, the entire study population must be?
must be at risk at the beginning of the time period
What are the pros of prospective cohort studies? (4)
- allow the estimate of incidence
- good for investigating multiple outcomes
- good for studying rare exposures
- reduced risk for survivor bias
What's the denominator of true rate?
What type of incidence would you use for this type of prospective cohort study?
What kind of cases are you looking at in prospective cohort studies?
What are the cons for prospective cohort studies? (5)
- selection bias (confounding)
- not an efficient choice for rare outcomes
- not good for outcome that takes a long time to develop
- loss to follow upexpensive
What are the pros of retrospective cohort studies? (4)
- less expensive than prospective cohort studies
- more efficient when examining rare outcomes
- may reduce information bias
What are the cons to retrospective cohort studies? (3)
- investigation limited to data that has already been collected
- information may not have been collected for research purposes
- exposed and unexposed groups may differ with regard to factors other than the exposure
Cohort study participants are recruited based on outcome status?
false, based on exposure status
True or false: prospective cohort studies can provide a direct estimate of risk
True, because rates of outcome are compared
What is an example of misclassification bias?
when the interviewer is aware of the exposure status of the study participants