Evidence of what the witness thinks, believes, or infers in regard to facts in dispute, as distinguished from personal knowledge of the facts themselves. The RULES OF EVIDENCE ordinarily do not permit witnesses to testify as to opinions or conclusions.
When this type of evidence is expressed by an expert witness, it may be used only if scientific, technical, or specialized knowledge will aid the trier of fact in understanding the evidence or determining a fact in issue. In the event that the witness is not testifying as an expert, the witness's testimony is restricted to opinions or inferences that are rationally based upon his or her perception and are helpful to a clear understanding of the testimony or the determination of a fact in issue.
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