What is the test for reversing a judgment in a personal injury case?

"An appellate court may not reverse a judgment because of the erroneous exclusion of evidence unless the 'substance, purpose, and relevance of the excluded evidence was made known to the court by the questions asked, an offer of proof, or by any other means.' [Citations.]" (People v. Livaditis (1992) 2 Cal.4th 759, 778.) The offer of proof requirement not only gives the trial court the opportunity to change its ruling in the event it is unclear how the evidence will assist the jury, it enables an appellate court to assess prejudice. (People v. Whitt (1990) 51 Cal.3d 620, 648.) Without an offer of proof, no evaluation of prejudice is possible.