How would you audit the automobile revenues (say $100 billion) of General Motors? Lets say that each car and truck has an invoice on the window and in GM's computer system, and that 8 million cars and trucks were sold. Then, how many invoices out of 8 million should you want to examine (audit) in order to verify the $100 billion of revenues in the financial statements? So, you, as the auditor, would feel comfortable to sign the auditors report (issuing your professional opinion as the auditor) attached to the GMs annual report to shareholders.