Final answer:
The test of shipping documents by an auditor is performed to assess the audit assertion of occurrence, confirming whether sales transactions actually happened and are properly recorded, thereby preventing revenue overstatements.
Step-by-step explanation:
An auditor selects a sample from the file of shipping documents to determine whether invoices were prepared. This test is performed to assess the assertion of occurrence. Occurrence is an audit assertion that addresses whether the transactions recorded in the financial statements have actually occurred in the period specified, and are related to the entity. Testing a sample of shipping documents to check if corresponding invoices were prepared is a way to gather evidence on the occurrence of sales transactions.
Since the invoices are directly tied to the shipping documents, this process helps confirm that sales transactions indeed took place and are valid. Essentially, it is a method to prevent overstatement of revenues by ensuring that each recorded sale reflects an actual shipment of goods.