There are generally two types of witnesses whose testimony may be admissible in evidence in a criminal trial: (1) fact witnesses who have personal knowledge of facts relevant to the prosecution or defense of the pending criminal charge; and (2) expert witnesses, who have expert knowledge that may assist the jury in determining a fact issue.

Relevant evidence—including witness testimony—is generally admissible, but the court may exclude relevant evidence if its probative value is substantially outweighed by a danger of one or more of the following: unfair prejudice, confusing the issues, misleading the jury, undue delay, or needlessly presenting cumulative evidence.

A two-part test generally governs whether expert testimony is admissible: (1) the expert must be qualified (must be an expert on the subject) and (2) the testimony must be relevant and be based on a reliable foundation. The judge makes the initial determination about whether the expert and the expert’s testimony meet these requirements. The judge has broad discretion to determine the admissibility of expert testimony, and the judge’s decision to admit or exclude the expert testimony evidence will generally only be reversed on appeal if there is an abuse of the judge’s discretion.

In Rhode Island, as in other jurisdictions, witness testimony in criminal trials can be broadly categorized into two types: fact witnesses and expert witnesses. Fact witnesses provide testimony based on their personal knowledge of the events related to the criminal case, while expert witnesses offer opinions based on their specialized knowledge, which can help the jury understand complex factual issues. The admissibility of evidence, including witness testimony, hinges on its relevance and whether its probative value is not substantially outweighed by risks such as unfair prejudice, confusion, misleading the jury, undue delay, or presenting redundant evidence. Expert testimony must pass a two-part test to be deemed admissible: the expert must have the necessary qualifications, and their testimony must be relevant and rest on a reliable basis. The presiding judge has considerable discretion in determining the admissibility of expert testimony, and an appellate court will typically only overturn such decisions if there is evidence of an abuse of discretion.