Cohort Studies Flashcards
What is important about all individuals in a cohort study at the beginning of the trial?
They must all be outcome free.
How do we compare the risks between the two groups in a cohort study?
We calculate the incidence rate ratio, and this gives us a measure of relative risk.
What is a prospective cohort study?
This is where outcome free individuals are recruited and then they are followed up a few years later.
What type of cohort study uses historical data?
Retrospective. In this case outcome free individuals are found and then reassessed at later records.
What is internal comparison in a cohort study?
This is where we compare rates between sub-cohorts as an IRR.
What is an external comparison in a cohort study?
This is where the rate observed in the study is compared with that of a larger reference population using SMR.
What is a lexis diagram?
This is a diagram which can be used to calculate a persons risk based on the different risks they will experience at different ages.
Will an internal or external comparison have a higher error factor?
An internal comparison because the sample size smaller.
State the relationship between sample size and error factor.
A larger sample size will reduce the error factor.
What is the healthy worker effect?
This is the effect which is caused because people who are working are likely to be healthy and able to work meaning there is a higher proportion of morbidity in the unemployed.
What are the limitations of external comparisons with cohort studies?
There can be limited data for the reference population, and there can also be selection bias because the populations are not comparable.
Name three ways in which cohort studies are better than case-control studies.
They are better for rare exposures, for studying a range of different outcomes and also for establishing that exposure preceeds outcome.
What is survivor bias in a cohort study?
This is where looses to the trial during follow up lead to a population which is no longer representative of the initial population and is therefore biased.
State two disadvantages of a cohort study
They are expensive and resource intense and they take a long time to obtain results.
What are the two different groups found in a cohort study?
Exposed and unexposed.
