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Law 101: Legal Guide for the Forensic Expert

Opinions and Conclusions

The expert should ensure that the attorney understands the distinction between objective (supported by science) and subjective (opinion) statements and uses them appropriately.

The attorney should also be made aware of the expert's assumptions and the presumptions he uses in formulating opinions, as opposed to facts supporting conclusions, or a combination of both (e.g., using presumptive tests to identify biological fluids).

An expert may testify like any other witness about facts about which the expert has personal knowledge, even if the expert was able to observe and assess the relevancy of those facts only because of special skill or experience.

An expert may also offer opinions related to facts about which the jury itself has insufficient experience to form an intelligent conclusion. The expert does not need to be certain of which facts the opinion is based on as a condition of its admissibility. It is sufficient that the testimony of a particular result is within the range of reasonable probability according to generally accepted scientific principles.

An expert should explain the limitations of results (e.g., two hairs being microscopically consistent does not mean that they originated from the same source).

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