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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
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What do you compare in cohort studies?
compare occurrence of disease in "exposed" and "unexposed" people
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Where do we get the exposure information from in a cohort study?
exposure information from BEFORE people develop the disease
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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
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What do you compare in prospective cohort studies?
compare the rates of the outcome in the exposed and unexposed groups
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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
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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
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How are the exposed and unexposed groups in retrospective cohort studies selected?
retrospectively, in the past
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What data does retrospective cohort studies use?
pre-existing data
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Where does the data from retrospective studies come from? (4)
- occupational databases
- hospital administration data
- health insurance databases
- health registries
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In retrospective cohort studies, how is disease occurrence usually measured?
cumulative incidence or incidence rate
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How is the relationship between exposed and unexposed groups in retrospective cohort studies quantified?
with RR (risk ratio)
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how can you prevent selection bias in retrospective cohort studies?
study groups should be similar in every respect other than the exposure
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What is information bias in retrospective cohort?
when information collected in exposed group (ex.type of info) differs from the information collected from unexposed
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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
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What is loss to follow up?
exposed individuals are more likely to be lost to follow up, compared with unexposed individuals
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Why might participants be lost to follow up? (3)
- death
- might move
- might lose interest
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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
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In prospective cohort study, the entire study population must be?
must be at risk at the beginning of the time period
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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
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What's the denominator of true rate?
person time
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What type of incidence would you use for this type of prospective cohort study?
incidence density
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What kind of cases are you looking at in prospective cohort studies?
incidence casess
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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 up
- expensive
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What are the pros of retrospective cohort studies? (4)
- less expensive than prospective cohort studies
- quick
- more efficient when examining rare outcomes
- may reduce information bias
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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
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Cohort study participants are recruited based on outcome status?
false, based on exposure status
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True or false: prospective cohort studies can provide a direct estimate of risk
True, because rates of outcome are compared
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What is an example of misclassification bias?
when the interviewer is aware of the exposure status of the study participants
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