A party may refuse to disclose otherwise discoverable material by asserting a privilege. Fed. R. Civ. P. 26(b)(5). In a civil case involving a federal claim, privilege is governed by the federal common law, unless otherwise provided by the U.S. Constitution, a federal statute, or rules prescribed by the Supreme Court of the United States. See Fed. R. Evid. 501. 'Evidentiary privileges in litigation are not favored.' Herbert v. Lando, 441 U.S. 153. 175, 99 S. Ct. 1635, 60 L. Ed. 2d 115 (1979). A court should only create and apply a privilege if it 'promotes sufficiently important interests to outweigh the need for probative evidence.' Univ. of Pa. v. E.E.O.C., 493 U.S. 182. 189, 110 S. Ct. 577, 107 L. Ed. 2d 571 (1990) (quoting Trammel v. United States, 445 U.S. 40, 51, 100 S. Ct. 906, 63 L. Ed. 2d 186 (1980)).