Retrospective cohort studies
A retrospective cohort study is the study of choice for an outbreak in a small, well-defined population, such as an outbreak of gastroenteritis among wedding guests for which a complete list of guests is available. In a cohort study, the investigator contacts each member of the defined population (e.g., wedding guests), determines each person’s exposure to possible sources and vehicles (e.g., what food and drinks each guest consumed), and notes whether the person later became ill with the disease in question (e.g., gastroenteritis).
After collecting similar information from each attendee, the investigator calculates an attack rate for those exposed to (e.g., who ate) a particular item and an attack rate for those who were not exposed. Generally, an exposure that has the following three characteristics or criteria is considered a strong suspect:
1. The attack rate is high among those exposed to the item.
2. The attack rate is low among those not exposed, so the difference or ratio between attack rates is high.
3. Most of the case-patients were exposed to the item, so that the exposure could “explain” or account for most, if not all, of the cases.