 An expert witness, in England, Wales and the United States, is a person whose opinion by virtue of education, training, certification, skills or experience, is accepted by the judge as an expert. The judge may consider the witness's specialized scientific, technical or other opinion about evidence or about facts before the court within the expert's area of expertise, referred to as an expert opinion. The expert witnesses may also deliver expert evidence within the area of their expertise. Their testimony may be rebutted by testimony from other experts or by other evidence or facts. Typically, experts are relied on for opinions on severity of injury, degree of sanity, cause of failure in a machine or other device, loss of earnings and associated benefits, care costs, and the like. In an intellectual property case an expert may be shown to music scores, book texts, or circuit boards and asked to ascertain their degree of similarity. In the majority of cases, the expert's personal relation to the defendant is considered and irrelevant. The tribunal itself, or the judge, can in some systems call upon experts to technically evaluate a certain fact or action, in order to provide the court with a complete knowledge of the fact slash action it is judging. The expertise has the legal value of an acquisition of data. The results of these experts are then compared to those by the experts of the parties. The expert has a great responsibility, and especially in penal trials, and perjury by an expert is a severely punished crime in most countries. The use of expert witnesses is sometimes criticized in the United States because in civil trials, they are often used by both sides to advocate differing positions, and it is left up to a jury to decide which expert witness to believe. Although experts are legally prohibited from expressing their opinion of submitted evidence until after they are hired, sometimes the party can surmise beforehand, because of reputation or prior cases, that the testimony will be favorable regardless of any basis in the submitted data. Such experts are commonly disparaged as hired guns.