Which of the following is NOT a criterion established in Rule 702?

Rule 702 of the Federal Rules of Evidence outlines the criteria for introducing expert testimony in legal proceedings. The focus of Rule 702 includes the qualifications of the expert, the relevancy of their testimony to the case, and the reliability of the methodologies used in arriving at their conclusions.

The criterion that is not established under Rule 702 is the application of principles to enhance witness credibility. While it is essential for expert testimony to be credible, Rule 702 does not explicitly address the enhancement of witness credibility as a criterion for establishing the admissibility of expert testimony. Instead, Rule 702 specifically emphasizes the necessity of the expert's knowledge aiding the understanding of evidence, the foundation of their testimony on sufficient facts or data, and the reliability of their methods.

In contrast, the other options align closely with the requirements set forth in Rule 702. Understanding evidence through expert knowledge, the necessity for adequate factual or data support for testimony, and the reliability of the methodologies are all explicit aspects of what makes expert testimony admissible in court.