A social work researcher has decided to do a study of people who have adopted children from outside the United States. He asks a friend, whom he knows to have recently adopted and asks if he would consent to an interview. He interviews his friend and then asks his friend if he could name other people who have recently adopted children from outside of the United States. His friend provides him with five names. The researcher contacts those five people, interviews them, and asks each of those people for names of other families. This sampling technique is known as: