The Supreme Court confirmed the legitimacy of this doctrine in "United States v. Nixon" in the context of a subpoena emanating from the judiciary, instead of emanating from Congress. The Court held that there is a qualified privilege, which once invoked, creates a presumption of privilege, and the party seeking the documents must then make a "sufficient showing" that the "presidential material" is "essential to the justice of the case". Chief Justice Warren Burger further stated that executive privilege would most effectively apply when the oversight of the executive would impair that branch's national security concerns. Regarding requests from Congress (instead of from the courts) for executive branch information, as of a 2014 study by the Congressional Research Service, only two federal court cases had addressed the merits of executive privilege in such a context, and neither of those cases reached the Supreme Court.