The volunteer effect is a form of selection bias, sometimes referred to as self-selection or volunteer bias. This phenomenon is based on the idea that individuals who volunteer to participate in epidemiological studies are different in some way from the target population that they have originated from. The result of this effect is that the resulting measure between exposure and disease is distorted.

The selection of the study population is a critical part of any epidemiological study. One of the methods of selecting individuals is to recruit volunteer participants from the target population. However, the main risk in this method is that this subset of the population differs in some way from the general population. It has been suggested that individuals who volunteer to participate in ...