Making comparisons is fundamental to epidemiological investigations and studies. We need to compare risk or rates of illness in exposed and unexposed group, or odds of exposure in cases and controls. Without making comparisons with a reference group, we cannot say from data analysis that an association with a given outcome is anything other than spurious. Such a reference group is designated as the control group in case control studies and the unexposed group in cohort studies. For the field epidemiologist, difficulties more often arise in choosing controls for case control studies than in choosing an unexposed group in cohort studies. This section will focus mainly on the former.

In order to define a control group, it is helpful to be clear about who the cases are, in other words,to start with a case definition. The case definition then helps to define the population from which the cases arise, the source population. This population is also the population from which controls should be drawn. The most important principle to follow is that controls should be representative of the source population. Cases can be defined in any way that the investigator decides, but this definition is key to determining the source population of cases, and hence the source population of controls.

There are many ways of choosing controls.This section reviews some of the more common types of controls, their advantages and disadvantages.