Expert witness
Hearsay rule

One important rule that applies to the expert witness but not the percipient witness is the exception to the hearsay rule. A percipient witness tells only what he/she actually knows about a case and nothing more. Percipient witnesses cannot give opinions nor conjecture regarding a hypothetical set of conditions. Conversely, the court does allow an expert to testify about issues that may not be personally known by them. This allows the expert to rely upon scientific articles, discussions with colleagues on the subject, testimony read in preparation for testimony in the case and similar pieces of information not personally known to the expert.
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