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quota samplea population SAMPLE selected by quotas from each defined portion of the population. The method of quota sampling does not fulfil the normal requirements of RANDOM SAMPLING. It involves breaking down the parent populations into strata (see STRATIFIED SAMPLING) according to relevant features (e.g. sex, age, social class, place of residence), and calculating how many individuals to include in each of these categories to reflect the parent population structure. At this stage randomness can be achieved, but once the size of each of these cells (i.e. the number of people of a certain sex, age and class living within a certain location) is decided, no attempt at randomness is made. Instead, the interviewers are instructed to achieve appropriate selections (quotas) to fulfil the requirements within each cell.
This lack of randomness in the selection of respondents means that though the interviewers achieve the correct proportion of the sexes, of age groups, of social class, etc., there is likely to be BIAS introduced on other VARIABLES since each member of the parent population has not had an equal chance of being chosen as a member of the sample (the criterion of randomness). MARKET RESEARCH and opinion polls commonly use this method for its cheapness and speed, but selecting a sample from individuals walking in town centres during daylight hours obviously risks biasing the sample on other variables than those specifically selected for.