Cohort studies Flashcards
What type of epidemiological studies are used to compare individuals?
observational: cohort studies, case control studies, cross sectional studies
interventional: randomised controlled trials
What type of epidemiological studies are used to compare populations?
time trends
ecological studies: geographical variations, age/sex patterns, social variations
What is a cohort study?
This study identifies a group of people and follows them over a period of time to see how their exposures affect their outcomes. This type of study is normally used to look at the effect of suspected risk factors that cannot be controlled experimentally, for example the effect of smoking on lung cancer.
What happens in a cohort study?
identify individuals
measure exposures in each individual
follow up individuals to determine disease occurrence
relate information on disease occurrence to exposure
What are the types of cohort study?
Concurrent
Historical
What is a concurrent cohort study?
collect exposure on cohort and follow up
What is a historical cohort study?
use pre existing exposure histories that have already been collected e.g. occupational exposure
What bias can occur in a cohort study?
loss to follow up
exposure usually measured at one point of time
selection of cohort
What are the disadvantages of a cohort study?
time consumption
requires a large group
expensive
What method of analysis is used for cohort studies?
Risk
Relative Risk
How is RISK measured?
Incidence is usually taken to be a measure of risk
Incidence: number of new cases (or deaths) of a disease in 100,000 people per year
How is relative risk measured?
Incidence of disease in exposed population/ Incidence of disease in unexposed population
What does a RR of 1 mean?
there is no difference between the two groups
What does a RR less than 1 mean?
the event is less likely to occur in the experimental group than the control group
What does a RR greater than 1 mean?
the event is more likely to occur in the experimental group than the control group
The relative risk of lung cancer amongst heavy smokers compared to non smokers is 24.5 ( 95% CI 16-38) p< 0.001 indicate?
pt be due to chance
The relative risk of lung cancer amongst heavy smokers compared to non smokers is 24.5 ( 95% CI 16-38) p<0.001. What does the confidence interval indicate?
95% confident that the true risk value is 16x higher or as high as 38x
What is a confounder?
confounder is a factor that is associated both with the exposure and the disease
What is survival analysis?
looks at how long someone lives - particularly with cancer
What is a Kaplan Meier curve used for?
used in survival analysis
looks at probability of survival over time
What are hazard ratios?
A measure of the relative probability of an event in two groups over time.
It is similar to a relative risk, but takes its account the fact that once people have certain types of event such as death, they are no longer at risk of that event.
A hazard ratio of 1 indicates that the relative probability of the event in the two groups over time is the same. A hazard ratio of more than or less than 1 indicates that the relative probability of the event over time is greater in one of the two groups.
If the confidence interval around a hazard ratio does not include 1, then the difference between the groups is considered to be statistically significant.
Give examples of cohort studies in the UK
British Doctors Study - male doctors
British Regional Heart Study - middle aged men
BUPA study - middle aged men
Give example of international cohort studies
Framingham heart study - USA
Nurse’s Health Study
What is absolute excess risk?
Risk in exposed - Risk in unexposed
used for the measure of importance
What is attributable proportion
Incidence in population attributable to exposure/ Incidence in population
p(relative risk - 1) / 1 + p(relative risk - 1)
where p= proportion exposed in population
