Political analysts are interested in perceptions of the current Democratic Party candidates in New York City. The number of registered Democrats and Republicans in New York is known. To form a sample of 1000, the analysts send out a team to sample Democrats and Republicans in proportion to the numbers registered. The team goes to major shopping malls and stops people as they enter, asking whether they are registered with one of the two parties. If so, the person is asked to fill out a questionnaire. The sampling technique used here would best be described as: