What is the legal test for due diligence in procuring a witness's attendance?

Properly considered, due diligence in procuring a witness's attendance appears to be a mixed question of law and fact. Mixed questions are those "in which the historical facts are admitted or established, the rule of law is undisputed, and the issue is whether the facts satisfy the [relevant legal] standard, or to put it another way, whether the rule of law as applied to the established facts is or is not violated." (Pullman-Standard v. Swint (1982) 456 U.S. 273, 289, fn. 19, 102 S.Ct. 1781, 1790, fn. 19, 72 L.Ed.2d 66.) Under this definition fall such questions as whether there were exigent circumstances to justify a warrantless search (United States v. McConney (9th Cir.1984) 728 F.2d 1195, 1199-1204 (in bank), cert. den., 469 U.S. 824, 105 S.Ct. 101, 83 L.Ed.2d 46), and whether there was founded suspicion to support an investigative stop (United States v. Maybusher (9th Cir.1984) 735 F.2d 366, 371, fn. 1, cert. den. (1985) 469 U.S. 1110, 105 S.Ct. 790, 83 L.Ed.2d 783). Under it appears to fall as well the question of due diligence: in addressing it the court must also determine the underlying facts, select the applicable legal standard, and apply the latter to the former.

The proper standard for mixed questions appears to be independent review. We clearly implied as much in People v. Leyba (1981) 29 Cal.3d 591, 174 Cal.Rptr. 867, 629 P.2d 961. There, without using the term "mixed question," we analyzed the scope of an appellate court's review of the trial court's determination of such a question--in that case specifically the reasonableness of an investigative stop. In considering a question of this kind, we explained, the trial court must engage in a two-step process, each step of which is subject to a different standard of review.