Internal audit is used by the top management to diagnose the health of the management system. Unfortunately it is also one process which is neglected by the top management.
External audits carried out by CB have to be completed in a short time based on one or at the most two samples.
On the other hand, in internal audits number of samples can be more to cover all the aspects of process being audited. Take an example: Inspection which has three aspects, incoming inspection, in-process inspection and final inspection. In the external audit all three inspections are audited at the same time.