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