The qualifications or characteristics necessary for a person to serve as a witness

United States v. Cerno, 529 F.3d 926, pin? (10th Cir. 2008) (conviction for aggravated sexual abuse in Indian country; defendant, invoking Fed.R.Evid. 403, contended that his conviction must be reversed because the district court improperly allowed the prosecutor to question...If a crime or act by a witness did not result in a criminal conviction but is probative of truthfulness and not unduly prejudicial, it may sometimes be used to impeach the witness.

Leonard v. Nationwide Mut. Ins. Co., 499 F.3d 419, 442 (5th Cir. 2007) (holding that the district court erroneously admitted statements made by insurer's agent to policyholders as "habit" evidence under Federal Rule of Evidence 406 because "evidence was lacking to suggest that [insurer agent's] statements...If a pattern of behavior is not invariable or almost invariable, it is not habit.

Fed. R. Evid. 405(b) ("In cases in which character or a trait of character of a person is an essential element of a charge, claim, or defense, proof may also be made of specific instances of that person's conduct.").Propensity, or character, is admissible if character is an essential element of a claim, charge, or defense.