GMP Auditing is a Science, and an Art. It is easy to read an FDA, PIC/S, MHRA GMP Guidelines, or a Procedure and check to see if they are being followed.
There are however, some things that are intuitive. GMP Auditors must decide how deep to probe, how many records to review, which GMP audit trail to follow and which to ignore. These decisions are difficult to make, and training alone cannot ensure GMP Auditors make the best decision. Even experience does not always equip the GMP Auditor to make the judgment calls required. Expertise is more than knowledge of the Guidelines, Directives and organization documentation. Expertise is more than just experience.
The learning curve for expertise can be costly. Outsourced GMP Auditors typically have greater expertise than in-house GMP Auditors. In order to perform GMP Audits, organizations need to take resources away from their regular tasks. Because of this, GMP Auditors are sometimes chosen for their availability, not their ability to perform the audit. Management tends to find the GMP Auditors that will be least missed during the audits. The selected GMP Auditors also must be trained, and that also consumes resources. Finding time to schedule a GMP Audit also drains resources.