Dr. chandler is a personality psychologist who is interested in studying the characteristics of people who report being abducted by ufos. she finds several people in an online support group for ufo abductees to participate and asks them if they can provide the names and contact information of other people who have also been abducted. upon contacting these new participants, she asks them to refer her to even more people they may know who have been abducted. this is an example of what kind of sampling?