It is undisputed that the presumption of imputed knowledge is uniformly rebuttable and may be overcome by a proper ethical screen when the issue arises in the context of government and former government attorneys. (In re Charlisse C. (2008) 45 Cal.4th 145, 162; Chambers v. Superior Court, supra, 121 Cal.App.3d at pp. 902-903.) Yet if ethical screening can, in any given case, be considered effective to screen a former government attorney in a private law firm, it gives rise to the question why screening cannot be equally effective to screen a private attorney in the same private law firm. The effectiveness of the screening process depends on the policies implemented by the law firm, not on the former employment of the screened attorney. In cases of a tainted attorney working in a government office, the courts have concluded the following policy considerations justify the use of only a rebuttable presumption of imputed knowledge: (1) public sector attorneys do not have a financial interest in the matters on which they work, so have less of an incentive than private attorneys to breach client confidences; (2) public sector attorneys do not recruit clients or accept fees, so have no financial incentive to favor one client over another; (3) disqualification increases the costs for public entities, raising the possibility that litigation decisions will be driven by financial considerations rather than the public interest; (4) automatic vicarious disqualification will restrict the government's ability to hire attorneys with relevant private sector experience. (City of Santa Barbara v. Superior Court (2004) 122 Cal.App.4th 17.) There is another context in which a rebuttable presumption of imputed knowledge--and therefore, the use of ethical screens--has been adopted, that of the tainted nonattorney employee. When a tainted nonattorney employee of a law firm, possessing confidential case information, moves to an opposing law firm, vicarious disqualification of the opposing law firm is not necessary if the employee is effectively screened. (In re Complex Asbestos Litigation, 232 Cal.App.3d at pp. 579, 593, 596.) The same rule applies to a nonretained expert, who is then employed by the opposing side. (Shadow Traffic Network v. Superior Court (1994) 24 Cal.App.4th 1067, 1084-1085.) Indeed, a rebuttable presumption applies within an expert firm, allowing an expert firm to ethically screen an expert who interviewed with a plaintiff from an expert retained by the defendant in the same matter. (Western Digital Corp. v. Superior Court (1998) 60 Cal.App.4th 1471, 1485.) In all of these situations --government employees, former government employees, nonattorney employees, experts, and expert firms--the presumption of imputed knowledge is rebuttable, not conclusive. Moreover, the use of a rebuttable presumption is not justified as a "necessary evil" in order to advance important policy considerations. Instead, the rebuttable presumption is accepted because it is believed that, under the proper circumstances, ethical screening can work. There is no legitimate reason to believe that the same screening could not work in the context of private attorneys at a private firm. For example, in City of Santa Barbara v. Superior Court, supra, 122 Cal.App.4th at page 27, the court upheld an ethical wall in a government office, and stated, "Like all attorneys, the tainted attorney knows that her participation and use of confidential information against a former client would subject her to a host of problems including tort liability and state bar discipline. Such conduct would be a recipe for financial and professional suicide. We are confident that an attorney's oath and the severe consequences that would inexorably flow from a breach thereof, coupled with an effective 'ethical wall,' are sufficient to safeguard the former clients' confidences and preserve the integrity of the judicial process."