Epidemiology: Cohort Studies

Question 1

Draw a contingency table with exposure, outcome and no outcome

Answer 1

Outcome status| O+ O-
———————————————
E+ | A | B
———————————————
E- | C | D

Question 2

How would you calculate the incident risk in an exposed group using a contingency table?

Answer 2

Incident risk in exposed= A/ (A+B)

Question 3

How would you calculate the incident risk in a non-exposed group using a contingency table?

Answer 3

Incident risk in unexposed= C/ (C+D)

Question 4

How would you calculate the relative risk or risk ratio (RR)?

Answer 4

Incident risk in E+/ incident risk in E-

Question 5

How would you interpret a risk ratio of;
A) >1
B) <1
C) =0

Answer 5

A. Exposure is harmful
B. Exposure is protective
C. Exposure has no effect

Question 6

How would you calculate the incidence rate?

Answer 6

Incidence rate (IR)= no of cases of outcome/ person years

Expressed per 1000 per year

Question 7

Define a cohort study

Answer 7

A longitudinal study which looks at a specific group of the population and the exposures they have encountered to then assertain if they get the desired outcome under study. Can be retrospective or prospective

Question 8

What are some of the benefits of cohort studies

Answer 8

No recall bias, if prospective then exposure precedes outcome so dont need to worry about temporality, can include factors for confounding, can study multiple outcomes

Question 9

Name some disadvantages to cohort studies

Answer 9

Large time investement, large sample sizes required, reproducibility is hard, loss to follow up