Subjective evidence is defined as a symptom, as only patients can discover them. The symptom checker may be run by the user/patient or with the assistance of medical personnel, as symptoms are, by definition, known to the patient only. In opposition, a sign is objectively determined by a medical professional. For example, blood coming out of the nose is a sign. It is apparent to the patient, physician, and others. Dizziness, low back pain, tingling in the feet or anxiety are all symptoms, as only the user/patient can perceive them. A symptom checker can be of different forms. The most frequent ones are either keyword based or artificial intelligence based. All symptom checkers will try to determine a differential diagnosis (DDX).