What is the test for denying a defendant a right to interview and present a witness?

This error does not, however, compel reversal. A defendant is denied his constitutional right to have compulsory process for obtaining witnesses in his favor when he is improperly precluded from putting on the stand a witness who is physically and mentally capable of testifying to events that he had personally observed and whose testimony would be relevant and material to the defense. (Washington v. Texas, supra, 388 U.S. 14, 23, 87 S.Ct. 1920, 1925, 18 L.Ed.2d 1019.) Thus, violation of the right to compulsory process or to due process based on deprivation of the right to interview or present a witness requires some showing that the evidence lost would be both material and favorable to the defense. (United States v. Valenzuela-Bernal (1982) 458 U.S. 858, 872-873, 102 S.Ct. 3440, 3449-3450, 73 L.Ed.2d 1193.)