Cohort studies. Flashcards
What is the best type of observational study?
A cohort study.
What simple exposure category is often used in cohort studies?
Yes/No.
Why is the final population in a cohort study slightly smaller than the original population?
Death/ loss to follow up.
When are cohort studies particularly useful?
When the time period between the exposure and the disease is relatively short.
Why can it be useful to use occupational populations (such as the Navy) as target populations?
They tend to be more compliant.
Name 6 types of target populations used in a cohort study.
- Population based on geographical region.
- Population based on a certain occupation.
- Population based on disease status.
- Population based on risk group.
- Population based on an exposure event.
- Population based on a birth cohort (eg those born in a certain year).
If a cohort study is small enough what can be used to encourage people to comply to follow up?
Payment.
What 4 methods can be used to assist with follow up?
- Get individuals to supply a next of kin.
- Use the national postal service for address updates.
- Social security records.
- Motor vehicle records.
Cut points used when categorising continuous data always need to have a biological significance. True or false?
False. They can be arbitrary.
If an outcome is subjective what do you need to do when carrying out a cohort study?
Define this outcome upfront.
What two types of outcomes can be measured in a cohort study?
- Single/ discrete events
2. Multiple occurrence events.
When looking at the outcome of a discrete event in a cohort study do you consider disease subtypes?
No.
Name two examples of discrete/single outcomes that can be looked at in a cohort study.
- Mortality.
2. First occurrence of a disease.
What transition is looked at in a cohort study when the outcome occurs multiple times?
The transition from healthy status to disease status.
What needs to be uniform in exposed and non exposed individuals in a cohort study to reduce bias?
Measures for detecting outcome.
Is it easier to ‘clean’ a case control or a cohort study?
Case control as can do at the time of collection.
What did the NIH-AARP diet and health care study investigate?
Cancer incidence in 500,000 50-70 year olds. Produced 53,000 incident cases.
How many publications have come from EPIC?
100+
What case control ratio was used in EPIC?
2:1.
Smaller studies can have less power. What is an advantage of using a smaller study?
Repeats of measurements can be taken.
If a cohort study is both retrospective and prospective what is it called?
Ambidirectional.
An advantage of cohort studies is the exposure is measured for the reason you want to in the way you want it to be. True or false?
False. This is not the case with retrospective cohort studies.
Retrospective studies are much quicker to carry out than prospective studies meaning they are more often used. True or false?
False. They are quite rare.
Future follow up can occur in a retrospective cohort studies although this is rare. True or false?
False. If this does happen this is an ambidirectional cohort study.
Why are retrospective cohort studies rare?
As the data is not collected for their specific purpose meaning it can be difficult to use.
Exclusion criteria of who to include in recruitment and analysis of cohort studies should be stated upfront. Name reasons why individuals may be excluded (5 things).
- No histology recorded.
- Insufficient data at endpoint.
- Prevalence of a different disease at baseline (still recorded and used to investigate questions not related to that specific disease).
- Factors which mean they can not be ‘at risk’ from your disease. (still recorded and used to investigate questions not related to that specific disease).
- Questionable data.
Do reference groups used in cohort studies tend to be internal or external?
Internal.
When would an external reference group be used in a cohort study?
When everyone in the cohort is exposed. The outcome being measured also has to be present in the external reference group.
When external references are used what is it hard to do?
Adjust for confounding.
Are external reference groups more commonly used in prospective or retrospective cohort studies?
Retrospective.
What is the equation for the 95% confidence interval for RR?
e^(lnRR+-(1.96x (B/A(A+B) + D/C(C+D)^1/2))
What is the equation for attrituble risk?
Incidence in exposed- Incidence in unexposed.
What sort of regression can adjust for potential confounding in cohort studies?
Multiple logistic regression.
When will an individual contribute to a survival analysis calculation?
When they still have the potential to become a case.
What sort of regression is used in survial analysis?
Cox proportional hazards regression- time to event used as the dependant variable.
What is information bias?
When different quality and/or quantity of information is collected from the exposed/non exposed group.
What would you call this type of bias?
’ Misclassification of exposure or disease status.’
Misclassification bias.
What would you call this type of bias?
‘Individuals who partake in trials tend to be more affluent and health conscious than the average person from your population meaning they can be unrepresentative of the population as a whole’.
Healthy worker effect selection bias.
What sort of validity is affected if the healthy worker effect bias is present?
External validity effected (Internal is not).
Are cohort studies good for rare diseases?
No.
Cohort studies should not be used if the exposure changes over time. True or false?
Yes, they can be. This is a disadvantage of cohort studies.
Can both survival curves and hazard ratios be calculated from cohort studies?
Yes.
What is an advantage of using observational studies?
They do not have the same ethical concern as analytical studies.
Can you use case control or cohort studies be used to prepare for RCT’s?
Cohort.
What sort of bias is reduced in cohort studies compared to case control studies?
Survival bias.
What is guaranteed in cohort studies (and not in case control)?
Exposure occurring before disease status.
