Cohort studies Flashcards
What type of study is a cohort study and how does this differ from a RCT?
A cohort study is an observational (prospective) study compared to a RCT which is an interventional (retrospective) study
What is meant by a cohort study - how are they carried out?
A cohort study is where you identify individuals i.e. the ‘cohort’ and you measure the exposure in each individual
You then follow up the individuals to determine the disease/disorder occurrence
You then relate the information on the disease occurrence to the initial exposure
SO these are carried out over a long period of time
What are the different types of bias that can arise in cohort studies?
Loss to follow up - alters the level of exposure
Exposure being measured at just one time point
Biased selection of the initial cohort
What are the disadvantages of cohort studies?
They take a long time
They require a lot of people
They can be very expensive
What is meant by ‘incidence’?
This is the number of people in which the factor you are measuring occurs within your sample population
Incidence is taken to be a measure of risk
How do you calculate relative risk?
Incidence of disease in the exposed population/incidence of disease in the unexposed population
What is the relative risk if the risk of the exposed sample and the unexposed sample is the same i.e. the factor has no effect?
RR = 1
What are the values that you look for in confidence intervals when looking at means and at risk?
When looking at means - CI is statistically significant if it does not include 0
When looking at ratios i.e. risk - the CI is statistically significant if it does not include 1
What is a confounder?
A confounder is a factor that is associated with both the exposure and also with the disease
SO may not be the cause for the disease - may just be very closely associated with the actual cause of the disease
What is the adjusted and unadjusted relative risk and what should be used?
Can adjust the relative risk to excuse the presence of the confounders
Journals and articles will usually always provide both an adjusted and an unadjusted risk - should use the adjusted risk
What can be used to assess survival analysis?
Kaplan Meier curves can be plotted - plots proportion of people surviving over time
Hazard ratios can be used rather than relative risk when talking about survival
What information is provided by a hazard ratio?
Gives the risk of dying at any time point in one group compared to the other
How can you measure the ‘importance’ of the results from a study?
Use the:
Absolute excess risk
Attributable population
What is the absolute excess risk (AER)?
Risk in the exposed group - the risk in the unexposed group
Remember - used to determine the importance of results comparing two factors
If the AER is greater then this means that this factor is more important/has a greater role
What is the attributable population?
How is this calculated?
This is the:
Incidence in the population attributable to exposure/incidence in the population
p (relative risk - 1)/
1 + p (relative risk - 1)
where p = proportion exposed in the population (20% = 0.2 = p)
