The court may call or appoint its own expert witness for several reasons:

• To select a neutral and impartial expert to educate the court or the jury about a particular aspect of scientific or technical evidence, testing or procedures that require further explanation by someone with special expertise.
• To review the testing procedures of other experts.
• To save the court time and expense.
• The court may appoint an expert witness for the defense when the defendant is indigent, and when the court in its discretion believes the case warrants expert testimony in the service of a just and fair trial.

Example 1: The court appointed a DNA expert to educate the jury in a criminal case in which DNA evidence was material. The expert was not hired by or associated with either the prosecution or the defense in the case.

The expert provided the jury with an impartial explanation of the fundamentals of DNA testing: what it is, how it works, and the fact that, in DNA cases, the test is a probability of non-exclusion of a particular person.

Lawyers for both the prosecution and the defense then questioned the court's DNA expert regarding the testimony. The truth or accuracy of the expert's neutral fact testimony may not have been in question. However, lawyers, as advocates for their respective parties, may have specific objectives regarding the court expert's testimony.

Parties may question the court's expert in the presence of the jury or in the jury's absence, at the court's discretion.

Example 2: In an effort to save time and expense, the court requested that the prosecution and defense in a criminal case submit a slate of names of expert witnesses approved by both sides. The court then selected one or more experts from the preapproved slate to testify in the case. Lawyers for both sides had an opportunity to question the court-selected experts.

A key aspect of this selection is that the expert serves the court as a neutral witness, and does not serve either party in a criminal matter.

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