- Cohort is a group of individuals having some common characteristics. These characteristics may be birth date, age, marriage date or smoking.
- Cohort study is called as prospective study which means “looking forward”.
- You know the risk and disease comes later.
- It is also called incidence study because healthy individuals become diseased after few years and become a new case.
- Cohort study is conducted only when you have evidence about the exposure and disease.
- When exposure is rare but incidence is higher in exposed e.g. exposure to uranium is rare but if exposed, cancer chances are very higher.
- When sufficient funds are available, cohort study is conducted.
- When follow up is easy or possible, cohort study is conducted. I.e. completion of study is necessary (at least for 25 years).
- It starts with people free of disease.
- Exposure Status is known at baseline.
- Outcome occurs after follow up.
- Exposure status
Considerations while selecting Group
- Exposed and non-exposed groups should be free of disease.
- Both groups should be comparable.
- Both groups should be equally susceptible.
- Diagnostic criteria for disease
Elements of Cohort Study
- Selection of study i.e. subjects
- Obtaining data from study subjects (obtain exposure status through interview, questionnaire and through mails). Medical tests should be performed to check some disease in these subjects.
- Comparison group is selected.
- Non-exposed group is your comparison group. It is called as internal comparison. It is difficult.
- External comparison is easy because you already know that who are smokers and who are non-smokers. Non-smokers are your comparison group.
- Compare with general population. We will take two groups. E.g. cancer in uranium miners and cancer in general population. In this, uranium miners are study group and general population is comparison group.
- You will follow the exposed and non-exposed for a particular period depending upon the period of the development of the disease. e.g. 10, 20, 25 years
- Drawbacks of follow up include death of investigator, deaths from study group, loss of follow up or non-availability of funds.
Two factors are calculated;
- Incidence Rate (IR)
- Relative Risk (RR)