Cohort studies

Question 1

What type of epidemiological studies are used to compare individuals?

Answer 1

observational: cohort studies, case control studies, cross sectional studies
interventional: randomised controlled trials

Question 2

What type of epidemiological studies are used to compare populations?

Answer 2

time trends

ecological studies: geographical variations, age/sex patterns, social variations

Question 3

What is a cohort study?

Answer 3

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.

Question 4

What happens in a cohort study?

Answer 4

identify individuals
measure exposures in each individual
follow up individuals to determine disease occurrence
relate information on disease occurrence to exposure

Question 5

What are the types of cohort study?

Answer 5

Concurrent

Historical

Question 6

What is a concurrent cohort study?

Answer 6

collect exposure on cohort and follow up

Question 7

What is a historical cohort study?

Answer 7

use pre existing exposure histories that have already been collected e.g. occupational exposure

Question 8

What bias can occur in a cohort study?

Answer 8

loss to follow up
exposure usually measured at one point of time
selection of cohort

Question 9

What are the disadvantages of a cohort study?

Answer 9

time consumption
requires a large group
expensive

Question 10

What method of analysis is used for cohort studies?

Answer 10

Risk

Relative Risk

Question 11

How is RISK measured?

Answer 11

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

Question 12

How is relative risk measured?

Answer 12

Incidence of disease in exposed population/ Incidence of disease in unexposed population

Question 13

What does a RR of 1 mean?

Answer 13

there is no difference between the two groups

Question 14

What does a RR less than 1 mean?

Answer 14

the event is less likely to occur in the experimental group than the control group

Question 15

What does a RR greater than 1 mean?

Answer 15

the event is more likely to occur in the experimental group than the control group

Question 16

The relative risk of lung cancer amongst heavy smokers compared to non smokers is 24.5 ( 95% CI 16-38) p< 0.001 indicate?

Answer 16

pt be due to chance

Question 17

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?

Answer 17

95% confident that the true risk value is 16x higher or as high as 38x

Question 18

What is a confounder?

Answer 18

confounder is a factor that is associated both with the exposure and the disease

Question 19

What is survival analysis?

Answer 19

looks at how long someone lives - particularly with cancer

Question 20

What is a Kaplan Meier curve used for?

Answer 20

used in survival analysis

looks at probability of survival over time

Question 21

What are hazard ratios?

Answer 21

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.

Question 22

Give examples of cohort studies in the UK

Answer 22

British Doctors Study - male doctors
British Regional Heart Study - middle aged men
BUPA study - middle aged men

Question 23

Give example of international cohort studies

Answer 23

Framingham heart study - USA

Nurse’s Health Study

Question 24

What is absolute excess risk?

Answer 24

Risk in exposed - Risk in unexposed

used for the measure of importance

Question 25

What is attributable proportion

Answer 25

Incidence in population attributable to exposure/ Incidence in population
p(relative risk - 1) / 1 + p(relative risk - 1)
where p= proportion exposed in population