Bringing medical evidence of torture to international tribunals can assist the trier of fact to understand or determine an issue in dispute. Thus, while ordinary witnesses may only testify about matters which they have observed or experienced, the role of the medical expert witness is to give an objective, impartial opinion based upon medical and professional experience and his/her knowledge of the subject. An expert may draw inferences for the tribunal in order to enlighten the judges on specific issues of a technical nature, e.g. torture-specific types of lesions, psychological impact and consequences of torture, etc. Because the expert opinion can have a significant effect on the outcome of the trial, international tribunals require that it is provided by reputable persons who derive their conclusions through scientific methods. The question of whether a person is an expert is one of law for the determination of the Trial Chamber. Five principle admissibility considerations are applied in varying degree at the international tribunals. First, the tribunals seek to determine whether the witness has sufficient knowledge and experience to entitle him or her to be considered as an expert who can assist the court. For the purposes of determining whether a witness meets this requirement, the judge will examine the witness’ former and present positions and professional experience and publication of scholarly articles. Second, the tribunals may inquire whether the expert’s knowledge is sufficiently recognised as credible by others in the field. The expert statement or report must meet the minimum standards of reliability, which involves proof or prima facie reliability. There must be sufficient information as to the sources used in support of the statements. The sources must be clearly indicated and accessible in order to allow the other party or the Trial Chamber to test or challenge the basis on which the expert witness reached his or her conclusions. In the absence of clear references or accessible sources, the Trial Chamber will not treat such a statement or report as an expert opinion, but as the personal opinion of the witness, and weigh the evidence accordingly. Third, the tribunals may determine that the expert’s testimony regards an inference that the judges are capable of drawing based on their general knowledge and common sense. 19