Quota sampling
Quota sampling is a non-probability method that fills fixed quotas for a population's subgroups, such as age or gender, without randomly selecting who fills each quota, so it cannot support formal confidence intervals.
See it move
A firm sets quotas of 300 respondents aged 18 to 40 and 200 over 40, matching the city's known age split, then interviewers fill each quota with whoever is willing at a shopping centre. Because no random draw picks who fills a quota, unlike stratified random sampling, the result cannot support a formally calculated confidence interval.
Check yourself
A market research firm samples 500 people from a city, setting quotas of 300 aged 18-40 and 200 aged over 40 to match known population proportions. Interviewers approach people at a shopping centre until each quota is filled, with no random selection within either age group. Which statement about the resulting sample is correct?