A trial court may issue a writ of administrative mandate if an agency has: (1) acted in excess of its jurisdiction; (2) deprived the petitioner of a fair hearing; (3) committed a prejudicial abuse of discretion. (Code Civ. Proc., 1094.5, subd. (b).) Because demotions and layoffs affect an employee's "fundamental vested right" in employment, a trial court exercises its independent judgment in reviewing an agency's findings of fact. ( Boctor v. Los Angeles County Metropolitan Transit Authority (1996) 48 Cal. App. 4th 560 [55 Cal. Rptr. 2d 861]; Duax v. Kern Community College Dist. (1987) 196 Cal. App. 3d 555, 560-562 [241 Cal. Rptr. 860].) Under the independent judgment test, the trial court may weigh the credibility of witnesses in determining whether the findings of the agency are supported by the weight, or preponderance, of the evidence. ( Governing Board v. Haar (1994) 28 Cal. App. 4th 369, 377 [33 Cal. Rptr. 2d 744]; Fukuda v. City of Angels (1999) 20 Cal. 4th 805, 808, 819-822 [85 Cal. Rptr. 2d 696, 977 P.2d 693]; 8 Witkin, Cal. Procedure (4th ed. 1997) Extraordinary Writs, 274, 286, pp. 1075, 1090; Code Civ. Proc., 1094.5, subds. (b), (c).) On appeal, we review the trial court's findings of fact to determine whether they are supported by substantial evidence on the whole record. ( Kazensky v. City of Merced (1998) 65 Cal. App. 4th 44, 52 [76 Cal. Rptr. 2d 356]; 8 Witkin, Cal. Procedure, supra, 274, p. 1075.) "Abuse of discretion is established if the agency has not proceeded in the manner required by law, the order or decision is not supported by the findings, or the findings are not supported by the evidence." (Ibid.) Under the substantial evidence test, 'The power of the appellate court begins and ends with a determination as to whether there is any substantial evidence, contradicted or uncontradicted,' to support the trial court's findings. . . . 'We must therefore view the evidence in the light most favorable to the prevailing [party], giving [it] the benefit of every reasonable inference and resolving all conflicts in [its] favor. . . .' " (Estate of Leslie (1984) 37 Cal. 3d 186, 201 [207 Cal. Rptr. 561, 689 P.2d 133], citations omitted.) "The focus is on the quality, not the quantity of the evidence. Very little solid evidence may be 'substantial,' while a lot of extremely weak evidence might be 'insubstantial.' " (Toyota Motor Sales U.S.A., Inc. v. Superior Court (1990) 220 Cal. App. 3d 864, 871-872 [269 Cal. Rptr. 647].) the testimony of a single witness may be sufficient. ( In re Marriage of Birnbaum (1989) 211 Cal. App. 3d 1508, 1513 [260 Cal. Rptr. 210].) Thus, " 'There might be foundational matters of fact with respect to which the trial court's findings would be conclusive on appeal if supported by substantial evidence. However, the ultimate questions, whether the agency's decision was . . . unlawful or procedurally unfair, are essentially questions of law. With respect to these questions the trial and appellate courts perform essentially the same function, and the conclusions of the trial court are not conclusive on appeal.' . . . That is, foundational factual findings must be sustained if supported by substantial evidence; however, the ultimate determination of whether the administrative proceedings were fundamentally fair is a question of law to be decided on appeal." ( Rosenblit v. Superior Court (1991) 231 Cal. App. 3d 1434, 1443 [282 Cal. Rptr. 819], citations omitted.) Put another way, we review questions of law de novo. ( Bostean v. Los Angeles Unified School Dist. (1998) 63 Cal. App. 4th 95, 107-108 [73 Cal. Rptr. 2d 523]; Riveros v. City of Los Angeles (1996) 41 Cal. App. 4th 1342, 1348-1350 [49 Cal. Rptr. 2d 238].)