A representative for Mrs. Jones Donuts stood outside of a grocery store with a clipboard, a survey form, and a table filled with donuts and small glasses of milk. The sign on the table said "FREE DONUTS." After each person picked up a donut and took a bite, she would ask, "Isn't that good?" Each positive reply, even if it was just a nod of the head, was entered on her clipboard sheet as a preference for that kind of donut. What kind of sampling is this?