Most research requires responses from certain groups, rather than an entirely random sampling of the population. In this situation, studies use quotas to cap certain categories of respondents, depending on the needs of the study. Perhaps the most common quota is a census representative sample, which forces the respondents to correctly mirror the population distribution of the country. The most common type of census representative quotas aim to have a sample with the correct proportions of age, gender, and region.

Consider a company that already knows that its product’s user base consists of 18–24 year olds. 80% are females, and 20% are males. When they consider the sampling criteria for an upcoming study testing users’ perceptions of a new feature, they might want to set quotas based on these gender distributions, as a 50/50 gender split would not accurately reflect their user base.