Discovery sampling is most effective for investigating the auditor's suspicion that the head of commercial lending has been granting loans without the required collateral. This sampling technique is designed to detect at least one occurrence of a specific characteristic (in this case, loans without collateral) within a population. It is particularly useful for identifying fraud or non-compliance with specific policies and procedures. Discovery sampling focuses on finding exceptions, making it suitable for the auditor's investigation into potential misconduct.
Reference: The IIA's Global Technology Audit Guide (GTAG) and The IIA's International Standards for the Professional Practice of Internal Auditing.