The airline company from Exercise 4, which is interested in the opinions of its frequent f lyer customers about its proposed new routes, has decided that different types of customers might have different opinions. Of their customers, 50% are silver level, 30% are blue, and 20% are red. The researchers first compile separate lists of silver, blue, and red members and then randomly select 5000 silver members, 3000 blue members, and 2000 red members to email. What kind of sampling method have they used?