Final answer:
Comparing fair value to cost does not assist auditors in testing the existence assertion for investment securities, as it relates to the valuation assertion rather than the confirmation of existence.
Step-by-step explanation:
In testing the existence assertion for investment securities, auditors use various methods to verify that these securities are actually held by the entity and are properly recorded. Answering the question about which method would not assist an auditor:
Physical examination is a method where the auditor inspects the securities documents to confirm their presence.
Confirmation with the issuer involves contacting the entity that issued the securities to verify their issuance and the entity's holdings.
Confirmation with the custodian is performed by verifying with the third-party holding the investments (custodial bank, for example) to ensure they are held on behalf of the entity.
However, comparing fair value to cost is not a procedure to test the existence of the securities, but instead it's more related to the valuation assertion, ensuring that the securities are recorded at the appropriate amount.