A person has the personal knowledge that a witness must have if and only if the evidence is sufficient to permit a reasonable trier of fact to conclude that the witness personally perceived the matter. The preliminary question of personal knowledge is governed by the standard of whether the evidence is sufficient to allow a reasonable person to conclude that the person has personal knowledge; the applicable standard is not the more demanding preponderance of the evidence standard.

United States v. Snow, 517 F.2d 441, 444 (9th Cir. 1975) ("We are aware, as was the trial court, that the inference sought to be drawn by the prosecution from the name tape might be false. Someone other than the....The constitutional requirement that proof of criminal guilt be shown beyond a reasonable doubt is not a requirement that proof of guilt be conclusive, completely certain, or beyond any doubt.

United States v. Sioux, 362 F.3d 1241, 1247 (9th Cir. 2004) (upholding trial court's admission into evidence of accused's similar sexual assault where "plain language of the rule permits admission of subsequent acts evidence to the same extent it permits the introduction of evidence tending to demonstrate...Federal Rule of Evidence 413 and comparable rules in a few States create an express exception to the prohibition against circumstantial use of character, the prohibition against the use of character to show conduct. Rules such as Rule 413 provide that in a criminal trial for sexual assault evidence of the defendant's commission of another offense or offenses of sexual assault is admissible.

See Babcock v. General Motors Corp., 299 F.3d 60, 66 (1st Cir. 2002) (affirming lower court ruling permitting plaintiff to introduce testimony that decedent had habit of using seat belt when driving car, where plaintiff had sought to advance theory that decedent had buckled seat belt before fatal...The habit of a natural person is admissible to show the person's conduct on a particular occasion.