Suppose one of the clinics had a fire, and some of the patient files were lost. From talking to the prenatal educator, you know that this clinic regularly sees members from a nearby Native American community. You want to make sure that you are including an adequate number of Native Americans. Before completing your sampling frame and starting the research project, you ask a current participant in the program who is a member of this Native American community if she knows of other members who have gone through the program. She gives you the names of four other mothers, who then collectively give you an additional 17 names. What type of sampling is this?