Cohort Study

1. Cohort study Presented By Dr. Fredrick Stephen P.G in Community Medicine MGMC&RI, Pondicherry

2. Definition Cohort study is a type of analytical study which is usually undertaken to obtain additional evidence to refute or support the existence of an association between suspected cause and disease . Synonyms Longitudinal study Panel study Prospective study Forward looking study Incidence study

3. What Is Cohort Ancient Roman legion, A band of warriors. A group of people who share a common Characteristic or experience within a defined time period e.g. age , occupation, pregnancy etc

4. INDICATION OF A COHORT STUDY When there is good evidence of exposure and disease. When exposure is rare but incidence of disease is higher among exposed When follow-up is easy, cohort is stable When ample funds are available When attrition is minimal.

5. Design Of A Cohort Study

6. Framework of a cohort Cohort Disease Total YES NO Exposed to putative aetiologic factor a b a+b Not Exposed to putative aetiologic factor c d c+d Incidence in exposed a/ a+b Incidence in unexposed c/ c+d

7. SELECTION OF COHORTS Both the cohorts are free of the disease. Both the groups should equally be susceptible to the disease Both the groups should be comparable Diagnostic and eligibility criteria for the disease should be defined well in advance Both the groups are followed up. Incidence among exposed is significantly higher than that of non exposed.

8. Elements Of Cohort Study Selection of study subjects Obtaining data on exposure Selection of comparison group Follow up Analysis

9. Types of Cohort Study Prospective cohort study Retrospective (historical) cohort study Combination of Retrospective and Prospective cohort study Nested Cohort

10. Cohort study Advantages Disadvantages Can often show temporality of relationship Less bias due to prospective evaluation of exposures Can evaluate multiple diseases can establish cause - effect good when exposure is rare We can find out incidence rate and Relative risk. losses to follow-up often requires large sample ineffective for rare diseases long time to complete expensive Changes in diagnostic criteria over time. Need motivated cohort of people who will be repeatedly evaluated

11. Biases in cohort study Analytic bias Cross over bias Surveillance bias Information bias Loss to follow up bias Interviewer bias Non-response bias

12. Examples of Cohort Studies

13. Thank you