Which of the following is not a factor that may lead to an investigation by the National Boards?

The distinction between the factors that can lead to an investigation by the National Boards is crucial for understanding the regulatory environment for practitioners. Holding multiple certifications is a common practice among professionals and, in itself, does not necessarily indicate any issues with a practitioner's competency or ethical standing. Multiple certifications can actually enhance a practitioner's qualifications and show a commitment to continual learning and skill improvement.

In contrast, the other options represent serious concerns that could prompt an investigation. Unsatisfactory practice methods or conduct may suggest non-compliance with professional standards, potentially endangering patient welfare. Additionally, any impairment that affects a practitioner's ability to perform their duties safely and effectively is a significant concern that would warrant investigation to protect both the practitioner and the public. Thus, while multiple certifications can be beneficial, they do not directly relate to the factors typically investigated by the National Boards.