Quota sampling is a non-probability sampling technique where individuals within non-overlapping groups are sampled until a quota (e.g. 20 in each group) is reached. The goal is to create a sample that reflects the same proportions of individuals as the entire population with respect to known characteristics or traits.
Quota sampling may result in unintentional bias. The selection within each quota is not random, potentially leading to over- or under-representation of certain subgroups.
Note that quota sampling differs from stratified sampling or multi-staged clustered sampling, both of which are probabilistic sampling methods.