Expert evidence is evidence of a scientific or technical character in regard to a matter that is outside the domain of ordinary experience and knowledge. The evidence is usually in the form of opinions or conclusions based upon facts that for the purposes of an opinion are assumed to be true, although it may be in regard to scientific facts. The expert is one who has had special training or opportunities in a particular subject that the ordinary witness has not enjoyed, and who has thereby acquired certain habits of judgment.that render his explanations and opinions in the field of his specialty valuable as guides and worthy of consideration
Hutchins, Harry B. "The Physician as an Expert." Mich. L. Rev. 2 (1904): 601-614, 687-711.