It is well established that '' 'the body of common law knowledge''' must be '' 'a source of guidance' ,,, in our interpretation of the Rules. Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U. S. 579, 588 (1993) (quoting United States v. Abel, 469 U. S. 45, 52 (1984) (quoting Cleary, Preliminary Notes on Reading the Rules of Evidence, 57 Neb. L. Rev. 908, 915 (1978))).
The Court has acknowledged that the Federal Rules of Evidence worked a change in common-law relevancy rules in the direction of flexibility. See Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U. S. 579 (1993). Article IV of the Federal Rules, which concerns relevance, liberalizes the rules for admission of relevant evidence. See id., at 587. The Rules direct the trial judge generally to admit all evidence having 'any tendency' to make the existence of a material fact 'more probable or less probable than it would be without the evidence.' Fed. Rules Evid. 401, 402. The judge may reject the evidence (assuming compliance with other rules) only if the probative value of the evidence is substantially outweighed by its tendency to prejudice a party or delay a trial. Rule ...