The U.S. Supreme Court and other courts have said that statements made in formal judicial proceedings are "testimonial" and are therefore subject to the constraints imposed on the use of testimonial hearsay in criminal trials by the Confrontation Clause of the Sixth Amendment to the U.S. Constitution.

Fed. R. Evid. 103(a) ("(a) Effect of erroneous ruling. Error may not be predicated upon a ruling which admits or excludes evidence unless a substantial right of the party is affected, and ... (2) Offer of proof. - In case the ruling is one excluding evidence, the substance of the evidence was made known to the court by offer or was...If a trial court is unwilling to accept a general offer of evidence, the proponent of the evidence must make a specific offer of evidence.

United States v. Sinclair, 109 F.3d 1527, 1536-1537 (10th Cir. 1997) ("Although Rule 602 provides that a witness's testimony must be based on personal knowledge, it 'does not require that the witness' knowledge be positive or rise to the level of absolute certainty. Evidence is inadmissible ... only if...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.

Cf. Mobil Exploration & Producing U.S. Inc. v. Cajun Construction Services, Inc., 45 F.3d 96, 99 (5th Cir. 1995) (holding, in an action for recovery of money, that the district court erred in refusing to consider plaintiff's evidence of defendant's routine practice for loading its trucks because...The routine practice of an organization is admissible under Federal Rule of Evidence 406 or a comparable State rule in order to show conduct on a particular occasion even if the routine organizational practice is volitional.