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When knowledge of a technical subject matter might be helpful to a trier of fact, a person having special training or experience in that technical field, one who is called an expert witness, is permitted to state his or her opinion concerning those technical matters even though he or she was not present at the event. For example, an arson expert could testify about the probable cause of a suspicious fire.
A person who testifies at a trial because she has special knowledge in a particular field. This entitles her to testify about her opinion on the meaning of facts. Non-expert witnesses are only permitted to testify about facts they observed and not their opinions about these facts. In family law trials, common expert witnesses include: Actuaries, who testify about values of spouses' pension plans for the purpose of dividing them at divorce; Child psychologists or development specialists, who testify about the best interests of the child when custody or visitation is in dispute; Appraisers, who testify about property values when the parties cannot agree, and; Career counselors, who testify about a homemaker's ability to return to the work force for the purpose of determining the amount and duration of alimony.