Cohort Studies
Cohort
Group of individuals *sharing same experience* followed up for specific period of time Examples: Group of smokers Occupational cohort of chemical plant workers
Cohort Studies
Incidences in the exposed group and the not exposed group are calculated *(Calculate Relative Risk for both groups)*
Retrospective Cohort Study = Non-concurrent Cohort = Historical cohort Study
Initiation of study occurs after occurrence of disease / outcome of interest *Subject to bias*
Prospective Cohort Study = Concurrent Cohort = Longitudinal Study
Initiation of study occurs before occurrence of disease. Groups of exposed and unexposed individuals are monitored over time to assess the development of disease.
*Cohort Studies* are also called:
Longitudinal studies Follow-up studies Incidence Studies
Cohort Studies
One of the most useful *observational study* Individuals are divided on the basis of presence or absence of risk factors, then they are followed over time to determine if they develop a specific outcome or disease.
Advantages of Cohort Studies
The temporal sequence between exposure and disease is clearly established. Can examine multiple outcomes of a single exposure. *True estimate of risk can be calculated - RR*
Disadvantages of Cohort Studies
Time Money - expensive Subject to loss of follow up
Hawthorne Effect
When *individuals participating* in a study *change behavior (mainly towards positive)* for a temporary period of time, due to the fact that *they are being observed*
Association between hyperlipidemia and coronary heart disease:
prospective study design using community cohort (Framingham Study)
Food handling and gastroenteritis:
retrospective study design
Lost to Follow -Up
A study that loses > 25% of participants is flawed
Bias in cohort study
Bias is less of a concern in cohort studies than in case-control studies. Recall bias rare, but *misclassification and selection bias can still occur*.
Steps in cohort study
Choose the design Selection of exposed group Selection of comparison group Follow up Analysis and interpretation