CASE: You are completing the audit of Curzon Pty Ltd. You read a note in the file from your assistant, Joe Burns, which states,

I selected a sample of 20 entries in the Sales Journal and vouched them to the supporting documentation (corresponding Shipping Order and Sales Order). I found no errors, so concluded that the controls over the occurrence assertion for sales were effective.

REQUIRED: Is Joe justified in forming the conclusion that he did? Give the reason for your answer.