And what is the criteria to select as internal Auditor?
Although it is not mandatory, the best guidance for selection and criteria of auditors is the material contained in ISO 19011. That document contains solid suggestions.
Please note that internal auditor selection is a critical issue. The majority of organizations that implement standards such as ISO 9001, have ineffective internal audit programs; one of the reasons being the fact that internal auditors are poorly selected. Good auditors need to understand the business processes of the organization and how they impact customer satisfaction and product quality. They must be excellent communicators and possess very good understanding of the
INTENT behind the ISO 9001 (or appropriate standard) requirements.
Have a look at
this presentation that I delivered a few years ago, about the top ten reasons why so many internal audit programs are ineffective.