Quota sampling may be locked as a special form of stratified sampling. In this method, the investigator is told in advance, the number of the sample units he is to examine or enumerate from the stratum assigned to him.
In the language of stratified sampling, the quota of the units to be examined by the investigator from the stratum assigned to him is fixed for each investigator. The sampling quotas may be fixed according to some specified characteristic, such as income group, sex, occupation, political or religious affiliations etc.
The choice of the particular units or individuals for investigation is left to the investigators themselves. They are merely given the quotas with the specific instruction to inspect (interview) a specified number of units from each stratum.