Cohort Vs Case-control Studies
- A cohort study is prospective – subjects are identified at the time of exposure to risks, and then followed up to see if they get the disease.
- A case-control study is retrospective – cases with the disease are evaluated to see if they were exposed to risks in the past, and compared
with controls who do not currently have the disease.
Cohort Studies
- A group of people are exposed to a proposed risk factor for a disease - what proportion end up developing the disease?
- How does this compare with a control group who were not exposed to the risk factor, and are any differences significant?
Cohort studies ask the questions:
- If I am exposed to the risk, what is my chance of developing the disease?
– In epidemiology terms: what is the incidence of disease? - How much greater are my chances of developing a disease if I have been exposed to a particular risk, compared with if I have not been exposed?
– In epidemiology terms: what is the relative risk?
Example:
- Linking Zika virus (the risk factor) to microcephaly (the disease) – note that one disease
often acts as a risk factor for another disease - A group of women in early pregnancy are monitored for Zika symptoms and IgM. Those with Zika form the test group, and those without form the control group.
- The proportion of babies with microcephaly in each group are recorded and compared.
Case-control Studies
- A group of people have a disease that resulted from a proposed risk factor - what proportion with the disease were exposed to the proposed risk factor?
- How does this compare with a control group who do not have the disease, and
are any differences significant?
Example:
- Linking microcephaly (the disease) to past exposure with Zika virus (the risk factor) during pregnancy:
- Babies with microcephaly form the case group, and those without the condition are the control group.
- The proportion of babies in each group whose mothers have a history of Zika during pregnancy are compared.