A potential source of distortion in any study in which the subjects volunteer rather than being sampled from the population (and through such sampling procedures the chances of an unrepresentative sample are reduced). There is considerable support for the atypicality of volunteers: generally they are more motivated and tend to perform at a higher level on most tasks than do control subjects.

(see also: science, experimental research, statistical evidence, control group, double-blind, experimenter bias, randomized assignment, confound/confounding, variable, correlation, Observer effect»Observer bias)