What is the burden of the proponent of evidence in a motion to overturn a finding that excluded evidence?

The rule of Evidence Code section 354 is necessary because, among other things, the reviewing court must know the substance of the excluded evidence in order to assess prejudice. (People v. Anderson (2001) 25 Cal.4th 543, 580-581.) Under California law, it is the burden of the proponent of evidence to establish its relevance through an offer of proof or otherwise. An offer of proof should give the trial court an opportunity to change or clarify its ruling and, in the event of appeal, would provide the reviewing court with the means of determining error and assessing prejudice. To accomplish these purposes, an offer of proof must be specific and must set forth the actual evidence to be produced

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