A researcher has decided to do a study of people who hunt in a nearby national forest. He asks a friend, whom he knows to be a hunter, if he would consent to an interview. He interviews his friend, and then asks his friend if he could name other people who hunted in the forest. 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 hunters. This sampling technique is known as: