A sample, usually of human beings, in which each investigator is instructed to collect information from an assigned number of individuals (the quota) but the individuals are left to his personal choice. In practice this choice is severely limited by “controls”, e.g. he is instructed to secure certain numbers in assigned age groups, equal numbers of the two sexes, certain numbers in particular social classes and so forth. Subject to these controls, which are designed to make the sample as representative as possible, he is not restricted to the contacting of assigned individuals as in most forms of probability sampling.