Juanita is conducting survey research for the Colgate-Palmolive Company. The company recently sent several cases of personal-sized toothpaste to Dr. Yankum as a part of a promotion. Juanita is surveying patients as they leave Dr. Yankum's office to ask if the dentist gave them any hygiene items to continue quality dental care. Juanita has discovered that practically every patient received Colgate and has concluded in her report that dentists prefer Colgate toothpaste over other brands. What flaw exists in Juanita's research?