Steve is auditing Black Widow Inc. In reviewing the sales ledger, Steve saw that Black Widow had sold 5,000 grappling hooks to Hawkeye Ltd. Steve reviewed the acknowledgement of receipt signed by Hawkeye and the manifest given to the carrier. He then verified each step along the paper trail, checking backwards, until he reviewed the original invoice of this sale to ensure that the date, price, quantity, and customer's name all match. This illustrates what type of auditing