Quota sampling is a method of sampling widely used in opinion polling and market research. Interviewers are each given a quota of subjects of specified type to attempt to recruit for example, an interviewer might be told to go out and select 20 adult men and 20 adult women, 10 teenage girls and 10 teenage boys so that they could interview them about their television viewing. ******************** It suffers from a number of methodological flaws, the most basic of which is that the sample is not a random sample and therefore the sampling distributions of any statistics are unknown.
An alternative to random sampling, often used in street surveys. For example, if each sex makes up 50% of the population, 50% of interviews must be with men and 50% with women. A random sample will get 50% of each, on average, but a quota sample will get 50% every time. Nevertheless, other things being equal, a random sample is more accurate than a quota sample.
With quota sampling, individual units in the population do not have a known chance of selection. It is a type of stratified sampling in which selection of sampling units within strata e.g. age, sex is done by interviewers on a non-random basis, controlled to some extent by quotas allocated to different strata. Each interviewer is given an assignment of interviews, specifying how many of them are to be with men and how many with women, how many with people in various age groups and so forth. These quotas are calculated from available data (e.g. Census) to arrange that, for the sampling unit (ED or town or Constituency), the sexes, age groups and social class are represented in the sample in the right proportions.