What is the test for a trial judge to question a witness in a civil case?

A trial judge may question witnesses to clarify and develop facts in a nonprejudicial manner, but must not adopt a partisan stance. Rogers v. United States, 609 F.2d 1315 (9th Cir. 1979); United States v. Eldred, 588 F.2d 746 (9th Cir. 1978). The trial judge here did not overstep. Three questions were asked, each was pertinent, and all were designed to clarify the facts. In addition, the court advised the jury that its questions were formulated to help the record, and were not indicative of judicial opinion. On these facts, the court did not demonstrate bias or usurp the role of the prosecutor.