What is the impact of the Sixth Amendment violation beyond a reasonable doubt?

We examine whether the Sixth Amendment violation was harmless beyond a reasonable doubt. The factors to consider in conducting this analysis are: "the importance of the witness' testimony in the prosecution's case, whether the testimony was cumulative, the presence or absence of evidence corroborating or contradicting the testimony of the witness on material points, the extent of cross-examination otherwise permitted, and, of course, the overall strength of the prosecution's case." (Delaware v. Van Arsdall (1986) 475 U.S. 673, 684.)