How do you carry out a Cohort study?

Participants in a cohort study are grouped according to their

exposure status. In many cohort studies two groups of people

are compared, namely an “exposed group” and an “unexposed

group”. For example, comparisons might be made between

children who have received the MMR vaccine (exposed group)

and children who haven‟t received the MMR vaccine

(unexposed group). Or comparisons might be made between

smokers (exposed group) and non-smokers (unexposed group).

However, you will also encounter studies where more than two

groups will be compared and where researchers divide study

participants into a number of groups with different levels of

exposure. For example, in a cohort study investigating the

association between caffeine consumption during pregnancy and

miscarriage, women were categorised into three groups:

a) women who consumed ≥ 200mg caffeine/day, b) women

who consumed <200mg caffeine/day, c) women who consumed

0mg caffeine/day.

In a cohort study, the groups being compared will be „followed up‟ over a period of time and the incidence of the outcome of

interest will be compared between the groups e.g. researchers

might compare the incidence of autism between groups of

children who have and haven‟t received the MMR vaccine; or

they might compare the incidence of lung cancer between

smokers and non-smokers; or they might compare the incidence

of miscarriage between women with different levels of daily

caffeine consumption etc.6

Two incidence measures can be used to compare the groups:

a) the risk (cumulative incidence) of the outcome of interest can

be calculated in the „exposed‟ and „unexposed‟ groups. The

relative risk can then be calculated by dividing the risk of the

outcome in the exposed group by the risk of the outcome in the

unexposed group.

b) The „incidence rate‟ of the outcome of interest can be

calculated in the „exposed‟ and „unexposed‟ groups (e.g. we can

compare the incidence rate of lung cancer in smokers and nonsmokers). The “incidence rate ratio” can then be calculated by

dividing the incidence rate of the outcome in the exposed group

by the incidence rate of the outcome in the unexposed group.

(you will learn about “incidence rate ratiOS” during your lectures

next year).