After ISO 9001:2015 the requirement for the independence of internal auditors has been removed from the internal auditor definition but not from the audit definition. Now, in addition to meeting your organization's competence criteria, internal auditors only have to ensure objectivity and impartiality. Thus, the manager could audit his own subordinates during an internal audit if, and only if he/she can ensure that it will be carried as a systematic, independent and documented process for obtaining objective evidence. This requirement is lost when the auditor (the manager) is a person who could be affected by the audit. For example, as manager he/she who would need to deal with the corrective actions that were found. So, avoid having managers auditing their own subordinates during an internal audit.
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