For cohort studies, the field epidemiologist is likely to be involved in retrospective studies. In other words the investigation takes place after both exposure and disease have occurred. The commonest situation is an outbreak of food poisoning after a clearly defined event such as a party or wedding.  Following the same principles as for the case control study, it is first essential to define the source population.  This population then forms the cohort, usually defined as those who attended the function in question.  Individuals within the cohort are then classified into exposed or unexposed, for example,  according to whether they ate or did not eat specified items of food or drink. The unexposed constitute the reference group for each item.

Questions may arise about whether the unexposed should include those who did not eat any food. As for case control studies, this depends on your definition of the source population. Is the cohort defined as everyone who attended or everyone who attended AND who ate something?  The answer will be influenced by the hypothesis that we wish to test.  As the number who did not eat anything will probably be small, it may be sensible to include them. If we should discover a substantial proportion of cases among those who attended but did not eat any food, food may not be the source of the outbreak. 

What happens if everyone ate the food in question i.e. there is no unexposed group? Luckily for the epidemiologist, our investigations involve human behaviour which usually offers a rich variety of exposures. In a food borne outbreak where everyone ate the delectable tiramisu, we then rely on trying to measure different levels of exposure (different amount of Tiramisu consumed). The reference group then becomes those with the lowest level of exposure.