Evidence Code section 1040 provides in pertinent part: (a) As used in this section, 'official information' means information acquired in confidence by a public employee in the course of his or her duty and not open, or officially disclosed, to the public prior to the time the claim of privilege is made. (b) a public entity has a privilege to refuse to disclose official information, and to prevent another from disclosing official information, if the privilege is claimed by a person authorized by the public entity to do so and: (1) Disclosure is forbidden by an act of the Congress of the United States or a statute of this state; (2) Disclosure of the information is against the public interest because there is a necessity for preserving the confidentiality of the information that outweighs the necessity for disclosure in the interest of justice . . . . In determining whether disclosure of the information is against the public interest, the interest of the public entity as a party in the outcome of the proceeding may not be considered." The official information privilege in Evidence Code section 1040, subdivision (b)(2), is expressly conditional, not absolute. If the public entity satisfies the threshold burden of showing that the information was acquired in confidence, the statute requires the court next to weigh the interests and to sustain the privilege only if " 'there is a necessity for preserving the confidentiality of the information that outweighs the necessity for disclosure in the interest of justice.' " (Shepherd v. Superior Court, supra, 17 Cal. 3d at pp. 123-125; see PSC Geothermal Services Co. v. Superior Court (1994) 25 Cal. App. 4th 1697, 1714 [31 Cal. Rptr. 2d 213]; Rubin v. City of Los Angeles (1987) 190 Cal. App. 3d 560, 585-587 [235 Cal. Rptr. 516]; CBS, Inc. v. Block (1986) 42 Cal. 3d 646, 656 [230 Cal. Rptr. 362, 725 P.2d 470].) A trial court commits error under this section if the court fails to make the threshold determination or fails to engage in the process of balancing the interests. (Shepherd v. Superior Court, supra, 17 Cal. 3d at p. 125; PSC Geothermal Services Co. v. Superior Court, supra, 25 Cal. App. 4th at p. 1714; Rubin v. City of Los Angeles, supra, 190 Cal. App. 3d at p. 586.) In rare instances, courts have declared a privilege compelled by constitutional considerations even where not specifically provided by statute. (Valley Bank of Nevada v. Superior Court (1975) 15 Cal. 3d 652, 656 [125 Cal. Rptr. 553, 542 P.2d 977] [state constitutional right to privacy compelled limited privilege against disclosure of bank customer records despite exclusivity of the Evidence Code on the subject of privileges]; Mitchell v. Superior Court (1984) 37 Cal. 3d 268, 274-284 [208 Cal. Rptr. 152, 690 P.2d 625] [First Amendment considerations compelled qualified privilege of newspaper reporter to refuse to disclose sources]; see also Welfare Rights Organization v. Crisan (1983) 33 Cal. 3d 766, 769 [190 Cal. Rptr. 919, 661 P.2d 1073, 31 A.L.R.4th 1214] [unless a privilege is expressly or impliedly based on statute, its existence may be found "only if required by constitutional principles, state or federal"; statutory basis found]; Roberts v. City of Palmdale (1993) 5 Cal. 4th 363, 373 [20 Cal. Rptr. 2d 330, 853 P.2d 496] [courts may not add to the statutory privileges "except as required by state or federal constitutional law"; statutory basis found].) But even in those rare cases where constitutional considerations require judicial declaration of a privilege not based on statute, the courts have utilized a balancing of the interests approach, and eschewed creating an absolute privilege. In Valley Bank of Nevada v. Superior Court, supra, 15 Cal. 3d at pages 657-658, the court balanced the constitutional interest of bank customers to maintain reasonable privacy regarding their financial affairs against the right of civil litigants to discover relevant facts. The court "readily acknowledge[d] that relevant bank customer information should not be wholly privileged and insulated from scrutiny by civil litigants," noting that " 'in order to facilitate the ascertainment of truth and the just resolution of legal claims, the [civil discovery statute] clearly exerts a justifiable interest in requiring a businessman to disclose communications, confidential or otherwise, relevant to pending litigation.' " In Mitchell v. Superior Court, supra, 37 Cal. 3d at page 276, the court weighed the First Amendment values furthered by protecting the confidentiality of a news reporter's sources against "the parallel importance of the policy favoring full disclosure of relevant evidence." It concluded, "there is neither an absolute duty to disclose nor an absolute privilege to withhold, but instead a qualified privilege against compelled disclosure which depends on the facts of each particular case." (Ibid.)