Evidence of habit may take the form of specific instances of conduct, opinion, or any other type of evidence otherwise permitted by the Federal Rules of Evidence.

Victor v. Nebraska, 511 U.S. 1, 17 (1994) ("Finally, Sandoval objects to the portion of the charge in which the judge instructed the jury that a reasonable doubt is 'not a mere possible doubt.' The Cage instruction included an almost identical reference to 'not a mere possible doubt,' but we did not...The reasonable doubt that requires an acquittal is a reasonable doubt rather than a merely possible, speculative, conjectural, or fanciful doubt.

United States v. Blankenship, 923 F.2d 1110, 1116-1117 (5th Cir. 1991) (trial court has broad discretion to declare witnesses to be incompetent but trial court did not abuse its discretion in ruling that the challenged witness was competent).It is possible that persons may be disqualified from serving as witnesses on grounds not expressly enumerated in codes of evidence or by general common law principles -- on unenumerated grounds such as age, illness, sensory defects, defects in intelligence, lack of understanding, inability to communicate, defects in memory, and inability to be truthful.

Cf. United States v. Fleck, 413 F.3d 883, 890-891 (8th Cir. 2005) (holding that even though the evidence of Fleck's alleged insurance fraud was inadmissible where it failed to "provide any context for and were not inextricably intertwined - indeed they had nothing to do with - the commission of the...Evidence that helps tell a factual story in an intelligible way or that puts the evidence or the issues in the case in a meaningful or intelligible context is "relevant."