One of the most challenging aspects of my job as a public accountant and auditor is explaining to clients why I’d like to do what I need to do.
Frequently you’ll find in your position as the auditor that client employees will enjoy seeing you twist in the wind and will find the most effective way to cause this is to ask you to justify the test you’re performing.
To juniors, this is kryptonite. They won’t be able to. They simply don’t have a clue. The keeners will try, and they might get far, but to the seasoned client employee, they will always be able to stump the junior auditor.
The key to counteracting this situation is to fall back on the assertions. Existence, completeness, accuracy, valuation. (There are others, but those four are key.) Explain how the test relates to the assertions for the given balance or transaction stream being tested.
This has two possible effects (both good): The client will understand why you’re doing the test, or they will have a better way to test the assertions you’ve mentioned.
And if you can explain the test to the client with reference to the assertions being tested, then you’ll better understand the test itself and be able to perform it more efficiently and identify errors if encountered. It’s a win-win.