The trial court must act as an independent gatekeeper. The trial court can admit expert or scientific evidence only if it finds that scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise, if (1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case.

United States v. Meacham, 115 F.3d 1488, 1492 (10th Cir. 1997) (upholding trial court's admission of witness's testimony about accused's prior act of molesting two of his stepdaughters where the court, "in ruling on the admissibility of this evidence, focused on the intent and weighed the prejudicial...A trial court can exclude evidence of another instance or instances of child molestation in a criminal trial for child molestation or child sex abuse if the trial court finds that the harm or potential harm of such evidence of another instance or instances of child molestation or sex abuse substantially outweighs the probative value of the evidence. A trial court has this power even if the evidence of another instance or instances of child molestation or child sex abuse is made presumptively admissible by a rule such as Federal Rule of Evidence 414.

Pastor v. State Farm Mutual Automobile Insurance Co., 487 F.3d 1042, 1045 (7th Cir. 2007) (upholding trial court's exclusion of evidence of change in insurance policy where plaintiff, in violation to Rule 407, used such evidence to establish defendant's culpable conduct).A subsequent remedial measure is not admissible to prove negligence, culpable conduct, a defect in the product or its design, or a need for warning or instruction.

Cf. United States v. Owens, 484 U.S. 554, pin? (1988) (meaning and constitutionality of Federal Rule of Evidence 801(d)(1)'s requirement that declarant be "subject to cross-examination concerning the [declarant's] statement").The hearsay exceptions created by Federal Rule of Evidence 804 or a comparable State rule apply only if the declarant (the maker of the statement) is unavailable to testify at the trial or proceedings at which the declarant's statement is offered.