Expert and Forensic Evidence
Expert opinion evidence may be admissible at trial based on relevance, necessity, policy, and qualification. The function of an expert is to provide the judge or the jury with a ready-made inference which the judge and jury, due to the technical nature of facts, are unable to formulate. If the judge or jury can form their own conclusions without aid of an expert, then the expert opinion is unnecessary.
Expert opinion is usually held to a higher level of scrutiny. An expert must be qualified and credible and criminal lawyer or prosecutor may have to give advance notice to the court that they will be bringing an expert to trial. The expert should be available for cross-examination on the issue/science of this testimony, as well as on their credentials.
In the United States, the judge acts as a gatekeeper for expert opinion. Judges generally use the Daubert Standard  to determine whether evidence is admissible, though some courts still use the Frye Test.
- Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579(1993)
- Frye v. United States, 293 F. 1013 (D.C. Cir. 1923)