Elementary Statistics (1+1)

3.1.8.2 Quota sampling

Quota sampling is a type of judgment sampling and is perhaps the most commonly used sampling technique in non-probability category. In a quota sample, quotas are set up according to some specified characteristics such as income groups, age, political or religious affiliations, and so on. Each interviewer is then told to interview the population units which constitute his quota. Within the quota, the selection of sample items depends on personal judgment. For example, in a radio listening survey, the interviewers may be told to interview 500 people living in a certain area and that out of every 100 persons interviewed 60 are to be housewives, 25 farmers and 15 children under the age of 15. Within these quotas the interviewer is free to select the people to be interviewed. The cost of conducting interview per person interviewed may be relatively small for a quota sample. However, there are numerous opportunities for bias which may invalidate the results. For example, interviewers may miss farmers working in the fields or talk with those housewives who are at home. If a person refuses to respond, the interviewer simply selects someone else. Because of the risk of personal prejudice and bias entering the process of selection, the quota sampling is not widely used in practical work.

Quota sampling and stratified random sampling are similar in as much as in both methods the universe is divided into parts and the total sample is selected from all the parts. However, the two procedures diverge radically. In stratified random sampling the sample within each stratum is chosen at random. In quota sampling, the sampling within each cell is not done at random; the field representatives are given wide latitude in the selection of respondents to meet their quotas.

Quota sampling if often used in public opinion studies. It occasionally provides satisfactory results if the interviewers are carefully trained and if they follow their instructions closely. It is often found that since the choice of respondents within a cell is left to the field representatives, the more accessible and articulate people within a cell will usually be the ones who are interviewed. Slight negligence on the part of interviewers may lead to interviewing ineligible respondents. Even with alert and conscientious field representatives it is often difficult to determine such control category as age, income, educational qualifications, etc.