Like any other expert, the medical witness is brought into court to render an opinion upon technical issues involved in a case. Fundamental to the opinions or conclusions which the medical witness renders is a matrix of data learned, observed or related, both fact and opinion. These subsidiary items, taken together, are commonly referred to as the basis of expert testimony. Thus a doctor, in testifying on the cause of a patient's condition, for example, might refer to and rely upon what he has observed in examining the patient, upon what the patient has told him of his symptoms, and upon the results of medical tests performed upon the patient. He might add to this information which he has learned in medical school and in practice, information from medical texts, and even material that has come to him as part of the ensuing litigation.
Paul D. Rheingold,
The Basis of Medical Testimony,
15 Vanderbilt Law Review
Available at: https://scholarship.law.vanderbilt.edu/vlr/vol15/iss2/5